United States
Environmental Protection
Agency
Motor Vehicle Emission Lab
2565 Plymouth Rd.
Ann Arbor, Michigan 48105
EPA-460/3-81-022
September 1981
Air
c/EPA
Costs, Benefits and Methods of
Including Tire Inflation In State
Vehicle Inspection Programs
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EPA-460/3-81-022
COSTS, BENEFITS AND METHODS OF
INCLUDING TIRE INFLATION INSTATE
VEHICLE INSPECTION PROGRAMS
by
Booz, Allen & Hamilton Inc.
Transportation Consulting Division
4330 East-West Highway,
Bethesda, Maryland 20814
Contract No. 68-02-3509
EPA Project Officer: R. Bruce Michael
September 1981
Prepared For:
Environmental Protection Agency
Office of Air, Noise and Radiation
Office of Mobile Source Air Pollution Control
Emission Control Technology Division
Inspection and Maintenance Staff
Ann Arbor, Michigan 48105
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This report is issued by the U.S. Environmental
Protection Agency to convey technical information and data
to those interested in the subject matter. Copies are
available commercially from the National Technical
Information Service, 5285 Port Royal Road, Springfield,
Virginia 22161.
This report was furnished to the Environmental
Protection Agency by Booz, Allen and Hamilton, Bethesda,
Maryland, in fulfillment of Contract No. 68-02-3509. The
contents of this report are reproduced herein as received
from Booz, Allen. The opinions, findings, and conclusions
expressed are those of the author and not necessarily
those of the Environmental Protection Agency. Mention of
company or product names is not to be considered as an
endorsement by the Environmental Protection Agency.
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TABLE
0 F
CONTENTS
Page
Number
I. INTRODUCTION 1
II. FUEL ECONOMY AND TREADWEAR BENEFITS OF 4
TIRE INFLATION
III. SAFETY BENEFITS ASSOCIATED WITH TIRE 14
INFLATION
IV. INCORPORATING A TIRE INFLATION CHECK IN 28
EXISTING AND PLANNED INSPECTION PROGRAMS
V. METHODS OF MAINTAINING PRESSURE AFTER 48
INSPECTION
REFERENCES 54
APPENDIX A. CALCULATION OF FUEL A-l
ECONOMY AND TREADWEAR
BENEFITS
APPENDIX B. METHODOLOGY FOR ESTIMATING B-l
CHANGES IN PROGRAM COSTS
111
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I. INTRODUCTION
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I. INTRODUCTION
This report presents information to help states in-
clude a tire inflation check in their existing and planned
emissions, safety and combined safety and emissions
inspection programs. It has been estimated that as much
as 515 million gallons of gasoline are wasted in the
United States each year due to improper tire inflation.
Tire inflation if added to a state motor vehicle inspec-
tion program would help motorists increase vehicle fuel
economy and reduce tire wear as well as improve safety.
States adding tire inflation to inspection programs would
be able to reduce the effective cost of their programs.
BACKGROUND AND SUMMARY
Numerous surveys of the tire pressure of vehicles-
in-use have shown that many vehicles are being operated
with underinflated tires. It is estimated that if a tire
pressure inflation program were implemented in State motor
vehicle inspection programs a .36 to 2.2 percent fuel
economy benefit for the inspected fleet could be achieved
depending on the type of inflation program implemented
(see Table 1). As indicated in Table 1, tire inflation
would save each vehicle owner between $3.59 and $13.94 in
fuel costs per year. Additionally, correct inflation
would increase tire life, resulting in an additional
savings of $1.23 to $4.54 per year, as well as improve
vehicle safety. If the benefits of improved consumer
maintenance practices are considered, a tire inflation
program could save motorists as much as $28.72 per year.
An examination of available options for implementing a
tire inflation check in existing and planned centralized
and decentrialized inspection programs indicates that (1)
the cost per vehicle of adding a tire inflation check
would be about 70 cents, and (2) the test could be added
relatively easily. Thus the inclusion of a tire inflation
check in State vehicle inspection programs is both
beneficial and cost effective from the standpoint of the
motorist and State. The net savings for the motorist
(fuel economy and treadware savings minus additional test
cost) could exceed the total test fee of most inspection
programs ($5 to $10).
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TABLE 1. Summary of Program Benefits by Inflation Strategy
STRATEGY
FLEET FUEL ECONOMY BENEFIT
. Due directly to tire
inflation program
. Due to program and
improved consumer
maintenance .
FLEET TREADWEAR BENEFIT
. Due directly to tire
inflation program
. Due to program and improved
consumer tire maintenance
TOTAL SAVINGS
. Due directly to tire
inflation program
. Due to program and improved
consumer tire maintenance
INFLATE TO
MANUFACTURER'S
RECOMMENDATIONS*
Percent
.36%
.82%
1.87%
4.25%
Savings**
$3.59
$8.17
$1.23
$2.72
$4.82
$10.89
INFLATE TO
3 PS I ABOVE
MANUFACTURER'S
_RECOMMENDAT IONS *
Percent
.86%
1.55%
4.42%
7.99%
Savings
$8.47
H5.24
$2.82
$4.94
J11.29
$20.18
INFLATE TO
MAXIMUM
SIDEWALL
PRESSURE*
Percent
1.42%
2.24%
7.31%
11.56%
Savings
$13.94
$21.81
$4.54
$6.91
$18. 48
$28.72
* Adjusted for temperature.
** Expressed in terms of dollars per vehicle per year.
REPORT METHODOLOGY AND ORGANIZATION
This report is based on (1) a review of existing
information on the effects of tire pressure on fuel
economy, treadware and safety, (2) discussions with
numerous government and industry representatives
knowledgeable about tire inflation, and (3) data on the
characteristics and capabilities of existing and planned
inspection programs resident in Booz, Allen's files.
The report is organized into five chapters and two
appendices. Information on the fuel economy and treadware
benefits of tire inflation is presented in Chapter II, and
information on safety is presented in Chapter III.
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Chapter IV discusses the methods of measuring tire
inflation in both centralized and decentralized inspection
programs. Topics addressed include alternative inflation
strategies, equipment and manpower requirements and cost
per vehicle.
Since most inspection programs require inspection only
once a year, encouragement of vehicle owners to continue
to monitor tire pressure throughout the year .is impor-
tant. Thus, means of maintaining tire pressure after
inspection were investigated, and the results are pre-
sented in Chapter V.
Appendix A contains the methodology employed to
calculate the fuel economy and treadware benefits
presented in Chapter II, and Appendix B contains a step by
step methodology to aid program planners determine the
potential cost impact of adding a tire inflation check to
their programs.
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II. FUEL ECONOMY AND TREADWEAR BENEFITS OF
TIRE INFLATION
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II. FUEL ECONOMY AND TREADWEAR BENEFITS OF
TIRE INFLATION
This chapter examines the fuel economy and treadwear
benefits of proper tire inflation. It is divided into the
following sections:
Current State of the Light Duty Vehicle Fleet
Regarding Tire Inflation
Effect of Tire Pressure on Fuel Economy
Effect of Tire Pressure on Treadwear.
Information presented in this chapter shows that the total
savings per motorist per year due to a tire inflation
program will range from $4.82 to $28.72. The beneift due
to improved fuel economy is from $3.59 to $21.81 and the
benefit due to reduced tire treadwear is from $1.23 to
$6.91.
CURRENT STATE OF THE LIGHT DUTY VEHICLE FLEET REGARDING
TIRE INFLATION
Several studies have been made over the past few years
on the tire pressures of motor vehicles. In general,
these studies have indicated that tires on vehicles in the
light duty vehicle fleet are underinflated relative to
manufacturers' recommendations. Most notable among the
studies that were conducted are those by Viergutz et.
al. and the U.S. Environmental Protection Agency
(EPA). As will be described below, these two studies
indicate that depending on the type of tire and climatic
conditions, tires average 0.8 to 3.3 psi underinflated,
with an overall average underinflation of about 2.0 psi.
Viergutz Study
The Viergutz et al^- study was based on pressure
measurements from a sample of 2400 vehicles parked in the
Chicago metropolitan area over a summer and winter
period. Vehicles were immobile for at least three hours
before any pressure measurements were made. The results
of that sample for 15-inch diameter size H and up radial
tires on domestic automobiles in winter are shown in
Figure 1.
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12
N - 403
u - -2.7
3o - + 15.1
-17 -15 -13 -11 -9
-7 -5-3-1 1 3 5
Measured Pressure Differencials
11 13 15
17
FIGURE 1. Pressure Differential from Manufacturers'
Recommendation, Histogram and Estimated Normal
Distribution: Radial Tires, 15-inch Diameter,
Size H and Up, Domestic Autos, Winter. (Reference 1)
As shown by the figure, the histogram is quite peaked
near the mean, and the results indicate an average under-
inflation of approximately 2.7 pounds per square inch
(psi). These results are typical of that found by
Viergutz. For all tires measured in the sample, the
summer average underinflation was approximately 0.8 psi
and the winter average underinflation was approximately
2.5 psi.
U.S. EPA Study
The EPA^ study was based on a representative sample
of vehicle make/models in six different cities: Los
Angeles, Houston, Phoenix, St. Louis, Denver and
Washington, D.C. Tests were all made at a uniform cold
temperature of 76°F and corrected to reflect the mean
outside temperature for the month and location of the test
-------
Table 2 shows the results of the survey. Overall, the
average pressure was 2.0 psi less than manufacturers'
recommendations. About 6 3 percent of all tires were
underinflated, and of these, the average underinflation
was 4.5 psi.
TABLE 2. Results of EPA Survey
Front
Mean
Mean
Mean
Recommended
Pressure, psi
Measured Pressure, psi
Difference,
psi
25
24
1
.9
.6
.3
Rear
27.
24.
2.
3
5
8
Overall
26
24
2
6
6
0
Mean Pressure Difference of
Underinf lated Tires, psi
Percent Underinf lated
4.1
56.6
4.9
68.4
4.5
62.5
Source: References 6 and 12.
The EPA results agree quite well with the Viergutz
study and with the general results of other investigators.
All subsequent calculations and results presented in
this report are based on use of the EPA data. The EPA
data was selected for use because it is more representa-
tive of vehicles throughout the U.S.
EFFECT OF TIRE PRESSURE ON FUEL ECONOMY
Considerable data have indicated that higher inflation
pressure significantly reduces tire rolling resistance and
the energy dissipated in a vehicle's tires, leading to an
overall improvement in a vehicle's fuel economy. Pre-
sented below are the results of studies on this subject
plus a discussion of the potential fuel economy benefit of
incorporating a tire inflation check in a State vehicle
inspection program.
Fuel Economy Versus Rolling Resistance
The exact amount of rolling resistance improvement to
be realized from proper tire inflation is a function of
the amount of inflation change and the type of tires on
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the vehicle. Figure 2 shows the relationship between tire
pressure and tire rolling resistance* of a vehicle at 50
mph for different types of tires.
Condicion.s :
en': TRA Load
5(1 mph Spi-r-d
Lo.id 9
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TABLE 3. Effects of Tire Pressure on Fuel Economy
Researcher
Effect on Fuel
Economy*
Study
Parameters
Grugett et. al.6
.33% per psi
Goodyear?
Taylor-
.30% per psi
.75% per psi
.38% per psi
.5% per psi
Radial Tires, 1979
Chevy Novas, Com-
posite Urban-
Highway Driving
Cycle
Radial Tires, Full
Size Sedan, 45 mph
Constant Driving
Cycle
Bias Tires, Full Size
Sedan, 45 mph
Constant Driving
Cycle
Radial Tires, GM
X-Cars, Combination
City-Highway Driving
Cycle
Radial Tires, GM
X-Cars, 30 mph
Constant Driving
Cycle
*Percent Improvement per psi
the type of tire, specific vehicle characteristics and driv-
ing cycle. Measurements made on 1979 Chevrolets by Grugett
et alb indicated a 0.33 percent change in fuel consump-
tion for each psi pressure change for radial tires over a
composite urban-highway driving cycle. Goodyear made
direct fuel economy measurements of a full size sedan
travelling at 45 mph and found a 0.30 percent change in
fuel consumption for each psi for radial tires and a 0.75
percent change in fuel consumption for each psi for bias
tires.7/8 Finally, data presented by Taylor on GM
X-cars equipped with low rolling loss radial tires indi-
cated that over the 20 to 28 psi range, about a .38
percent change in fuel economy per psi occurs with the
combination city-highway driving cycle and about a .5
percent change in fuel economy per psi occurs for a
constant 30 mph driving cycle.^
In calculating fuel economy benefits for this study, a
fuel economy benefit of 0.33 percent per psi was used.
This value represents the benefit associated with a
-------
typical vehicle with radial tires driving over an
urban-highway driving cycle. This value was selected
because it is consistant in general with all studies.
Radial tires were assumed because they are becoming the
predominant tire on vehicles-in-use.
Potential Fuel Economy Benefits of Incorporating Tire
Inflation in a State Vehicle Inspection Program
Assuming an average underinflation of 2.0 psi and an
average effect on fuel economy of 0.33 percent per psi,
and given that the light duty vehicle fleet consumes 78
billion gallons of gasoline per year35 one can arque
that approximately 515 million gallons of gasoline per
year are needlessly consumed due to improper tire
inflation.
The fuel economy benefits associated with a specific
state tire inflation program depend on the pressure level
tires are inflated to during the pressure check. Fuel
economy benefits are presented in this section for three
different inflation strategies:
Inflating all tires to manufacturers'
recommendations, with temperature adjustment
Inflating tires to 3 psi above manufacturers'
recommendations, with temperature adjustment
Inflating all tires to the maximum pressure
indicated on the tire sidewall, with temperature
adjustment.
Chapter IV further defines and addresses the advantages
and disadvantages of each inflation strategy. Details of
calculations are presented in Appendix A.
A summary of the potential fuel economy benefits by
inflation strategy are presented in Table 4. Two sets of
results are presented for each inflation strategy:
The top row presents the benefits associated with
inflation during the vehicle inspection.
The bottom row shows additional benefits that
could be achieved if motorists improved their
tire maintenance practices as a result of
consumer information provided as part of the
inflation program (see Chapter V).
The table indicates that a motorist could save $3.59 to
$21.81 per year depending upon the inflation strategy and
the amount of benefit due to improved tire maintenance.
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TABLE 4
Summary of Fuel Economy Benefits by
Inflation Strategy
STRATEGY
INFLATE TO
MANUFACTURERS '
RECOMMENDATIONS*
INFLATE TO 3 PS I
ABOVE MANUFACTURERS'
RECOMMENDAT IONS *
INFLATE TO
MAXIMUM
SIDEWALL
PRESSURE*
Savings without
a consumer
information
benefit
$3.59
$8.47
$13.94
Savings includ-
ing a consumer
information
benefit
$8.17
$15.24
$21.81
* With temperature adjustment
The fuel economy results shown in Table 4 indicate that
significant savings could be achieved with an inflation
program. An inflation strategy using pressures greater
than those recommended by the manufacturer provides
considerably more benefit than one using manufacturers'
recommendations. In addition, a large benefit can be
achieved through an effective consumer information
program. Operational and safety implications of the
different inflation strategies are discussed in Chapters
III and IV.
EFFECT OF TIRE PRESSURE ON TREADWEAR
Fewer studies have been made of the effects of tire
pressure on treadwear than on the effects of tire pressure
on fuel economy. Nonetheless, sufficient evidence exists
to indicate, a benefit of proper tire inflation on tread
life. Presented below are the results of a recent study
conducted by B. F. Goodrich on the relationship between
tire pressure and treadwear plus a discussion of the
potential treadwear benefits of incorporating a tire
inflation check in a state vehicle inspection program.
10
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Relationship Between Tire Pressure and Treadwear
Recently, a series of wearout projections were made by
researchers at B. F. Goodrich Company based on tire wear-
out measurements taken on vehicles with different types of
tires running on a test track at different pressures. The
results are shown in Figures 3 and 4.
Figure 3 indicates that the wear of all three
tire constructions improves with higher inflation
pressures, leveling off at 32 psi.
Figure 4 indicates that pressure changes of only
a few psi change the shoulder/crown wear ratio*
of bias and bias-belted tires with, according to
the Goodrich report, unacceptable malwear
(decreased tire life due to uneven wear at the
pressure extremes of 16 and 40 psi. Pressure
changes have little effect on radial tire
shoulder/crown wear ratio.
Potential Treadware Benefits of Incorporating a Tire
Inflation Check in a State Vehicle Inspection Program
The Goodrich data shows approximately 1.7 percent
change in tire life for each 1 psi pressure change for
radials and a 2.4 percent change in tire life for each 1
psi pressure change for bias tires. However, the bias
tires also have a significant change in shoulder-crown
wear and may thus be more critically affected by tire
pressure than indicated by the average-groove data.
Using the result of radials and assuming an average
underinflation of 2.0 psi with 120,000,000 cars on the
road and an average tire life of three years, over
5,000,000 extra tires must be purchased each year due to
low tire pressure. According to NHTSA11, 8 gallons of
oil are needed to build a passenger car tire. Thus more
than 40,000,000 gallons of oil per year may be wasted due
to the low tire pressure effect on treadwear.
Appendix A presents some calculations of the reduced
tire wear benefits that could be achieved if a tire infla-
tion check were added to a state vehicle inspection pro-
gram. The results are presented in Table 5. Tire life
could be extended 2 to 10 percent (1,000 to 5,000 miles),
This is the ratio of the wear on the outside of the
tire to wear in the center of the tire.
11
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J 1
80-T-50
70-
60-
-40
50-
-30
'40-
i30-
20-
10-
o-Lo
-20
-10
16
110
A RADIAL
O BIAS-BELTED
O BIAS
32
165 220
INFLATION PRESSURE
40 PSI
275 kPa
FIGURE 3. Projected Treadwear Versus Inflation
Pressure: Average of All Grooves, 85% of
T&RA Design Load (Reference 13)
200
150
a 100
50
110
DBIAS
O BIAS-BELTED
A RADIAL
WEAR RATE @ 24 PSI (165 kPa) = 100 INDEX
24
32
-H h-
165 220
INFLATION PRESSURE
40 PSI
275 kPa
Note: Each Tire Type is Given the Value of 100 at 24 psi
FIGURE 4. Shoulder-Crown Wear Ratio
Versus Inflation Pressure
12
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and each motorist could save $1.23 to $6.91 per year
depending on the inflation strategy. If this amount is
added to the fuel economy savings, total savings per
motorist per year due to tire inflation in an inspection
program would range from $4.82 to $28.72.
TABLE 5
Summary of Treadwear Benefits by Inflation Strategy
Strategy
Inflate to
Manufacturers'
Recommendations*
Inflate to 3
psi above
Manufacturers'
Recommendations*
Inflate to
Maximum
Sidewall
Pressure*
Savings without
a consumer infor-
mation benefit
$1.23
$2.82
$4.54
Savings including
a consumer infor-
mation benefit
$2.72
$4.94
$6.91
With temperature correction.
13
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III. SAFETY BENEFITS ASSOCIATED WITH TIRE INFLATION
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III. SAFETY BENEFITS ASSOCIATED WITH TIRE INFLATION
In addition to fuel economy and treadware, there are
also a number of safety considerations associated with
proper tire inflation. This chapter examines the effects
of tire pressure on vehicle safety, specifically addres-
sing the following issues:
The relationship between low tire pressure and
vehicle accidents.
The effects of tire pressure on vehicle handling,
tire failure and front suspension system wear.
The information provided in this chapter indicates
that:
There are definite safety benefits in adjusting
vehicles with low tire pressure to the pressures
recommended by the manufacturer. The benefits
result from reduced chance of tire failure and
improved vehicle handling.
The same safety benefits resulting from inflating
low tire pressures to manufacturer recommended
pressures apply to inflating vehicles with low
tire pressure to pressures 2 to 3 psi over
vehicle specifications, since the evidence does
not indicate significant handling effects with
pressure changes in the 2 to 3 psi range.
Changes in handling are of concern as pressures
are increased further over vehicle specifica-
tions. However, some fleets have found that suc-
cessful vehicle operation is possible with tires
inflated to-the maximum pressure indicated on the
tire sidewall. Any changes in handling are not
considered problems by drivers. Pressures should
not be increased beyond the maximum sidewall
pressure to prevent any chance of tire failure.
Maintenance of manufacturer's recommended front-
to-rear pressure differences will in general
lessen any handling effect caused by inflating
tires to pressures higher than those recommended
14
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by the manufacturer. Eliminating front to rear
pressure differentials of 6 psi or more is likely
to result in a noticeable change in vehicle
handling.
There is some indication that pressures 4 to 6
psi higher than those recommended by the vehicle
manufacturers may cause excessive loads on the
suspension system of older vehicles.
Auto manufacturers' recommendations for tire
pressure are based upon research which balances
ride, handling, fuel economy and other vehicle-
related factors for a specific auto model. Since
these represent industry standards that can be
used in product liability litigation, manufac-
turers may hesitate to recommend overinflation to
improve one factor, such as fuel economy.
RELATIONSHIP BETWEEN LOW TIRE PRESSURE AND VEHICLE
ACCIDENTS
The National Highway Traffic Safety Administration has
estimated that low tire pressure is responsible each year
for 260,000 accidents, 720 deaths and 28,000 disabling
injuries.21 These estimates are based on a study con-
ducted by Indiana University in which an investigation of
hundreds of accidents identified low tire pressure as
being a probable or certain cause of accidents in .5 to
2.3 percent of the observed cases. A brief description of
this study and another study conducted by the Highway
Safety Research Institute (HSRI) is presented below.
While the HSRI study does not confirm NHTSA's estimate, it
does nonetheless suggest that tire maintenance practices
are poor and that improper tire matching and maintenance
practice are likely accident causation factors in crashes
involving wet or slippery roads.
Indiana University Study
A study in 1973 by Indiana University attempted to
ascertain the relative roles played by human, environmen-
tal and vehicular deficiencies in causing and increasing
the severity of automobile accidents.20 information was
gathered through on-site examination of hundreds of acci-
dents and an in-depth analysis of the cause of many of
these accidents was also undertaken. No instances of
blow-out or sudden tire failure were found during the in-
vestigation. In addition, in less than .2 percent of the
accident cases did any investigating team attribute low
tire pressure as a certain cause of the accident. Low
15
-------
tire pressure was sited as a probable or certain cause of
accidents in .5 to 2.3 percent of the observed cases,
depending on the investigation team and accident sample.
Highway Safety Research Institute Study
The Highway Safety Research Institute performed an
analysis of tire data collected on 518 vehicles involved
in accidents between September 1, 1975 and September 1,
1977.19 The study attempted to determine the frequency
of improper inflation and its relationship to accident
causation.*
The approach of the study was to compare the tire
pressures of randomly selected vehicles from Michigan
State Police checklane inspections conducted in the summer
of 1976 with the pressures of cars and light trucks in-
volved in accidents. Key findings of the study are as
follows:
No significant difference was found between the
overall inflation pressures of the accident and
control samples.
Tire pressure imbalances in the accident sample
were significantly greater than those in the con-
trol sample.
Accident vehicles that had the greatest imbalance
of tire pressures were those involved in crashes
on slippery roads.
The, study concluded that tire maintenance practices
are generally poor, but there is no evidence to implicate
poorly maintained tires as causative factors for accidents
on dry roads. However, improper tire matching and main-
tenance practices appear to be accident-causation factors
in crashes involving wet or slippery roads.
TIRE PRESSURE AND VEHICLE HANDLING
The effects of tire pressure on vehicle handling are
generally measured in terms of three factors:
Traction
Oversteer and Understeer
Aligning Torque.
The study also examined the effects of mismatched or
worn tires.
16
-------
Each is discussed below along with an examination of the
experience of certain fleets which have increased the tire
pressures on their vehicles above manufacturer's
recommendations as a means of increasing fuel economy.
Effect of Inflation Pressure on Traction
Tire traction is an important vehicle handling para-
meter. It is associated with a vehicle's ability to brake
and accelerate, and is particularly critical during wet
weather. In order to assess the effect of inflation pres-
sure on traction, Goodrich researchers ran wet and dry
traction tests at Automotive Proving Grounds in Pecos,
Texas. The results are presented in Figure 5. As shown,
in every case the data indicates that changes in inflation
pressures generally do not adversely affect wet or dry
traction.
Effect of Inflation Pressure on Vehicle understeer and
Oversteer
The effects of tire inflation on vehicle "understeer"
and "oversteer" have been investigated in several stud-
ies. 14'l^,16 understeer and oversteer refer to the
radius of the circle made by a turning vehicle compared to
the radius of the circle a vehicle would traverse accord-
ing to the angle of the front wheels (see Figure 6).
Thus, if a car turns in a larger circle than one would
expect given the turning angle of the front wheels, the
vehicle has "understeer." If the circle radius is smaller
than the angle of the front wheels would suggest, then the
vehicle has "oversteer."
Changes in vehicle understeer or oversteer will change
vehicle handling, especially during turns. However, a
safe level of understeer or oversteer cannot be defined.
Most American cars have significant understeer, and
changes in the amount of understeer would normally not be
detected by most drivers. For state motor vehicle
inspection programs, concerns center around not whether a
certain level of understeer is safe, but rather if:
Changes in understeer as a result of correct tire
pressure will improve vehicle handling.
Changes in understeer due to tire inflation
higher than that recommended by the vehicle manu-
facturer will be sufficient to be noticed by
motorists or cause excessive adjustments on the
part of motorists.
17
-------
g 1.0 r- "
O
Li-
Li-
UJ
8 0.5
g
^ 0
Q " '
BIAS
"- 16 24 32 40
1 1 1 1
1 1 1 1
110 165 220 275
INFLATION PRESSURE
21 n i " "
I.U
o
u_
8 0.5
0
t
<
o "
BIAS-BELTED
-16 24 32 40
1 1 1 1
1 1 1 1
110 165 220 275
INFLATION PRESSURE
S'-o
u
Ej
8 0.5
z
g
^
_
RADIAL
»- 16 24 32 40
J 1 -_ I I
1 O
48
i
1
PEAK
SLIDE
PSI
330 kPa
o
o
48
i
1
PEAK
SLIDE
PSI
330 kPa
o
48
1
PEAK
SLIDE
PSI
110 165 220 275 330 kPa
INFLATION PRESSURE
§1.0
o
s:
u.
80.5
z
o
JOEFFICIENT
3 _t
n o
o
lo
u. 0
51.0
o
K-
UJ
80.5
z
o
"-
BIAS
O- .Q o
16 24 32 40
1 I 1 I
1 1 1 1
110 165 220 275
INFLATION PRESSURE
BIAS-BELTED
o -o ° °-
o o "°
16 24 32 40
I 1 1 1
1 1 1 1
110 165 220 275
INFLATION PRESSURE
RADIAL
o o o °
16 24 32 40
1 1 I 1
1 1 1 1
110 165 220 275
INFLATION PRESSURE
a 20 MPH (32 km/h)
o 40 MPH 64 km/h
* 60 MPH (97 km/h)
48 PSI
1
1
330 kPa
020 MPH (32 km/h)'
O 40 MPH (fid km/hi
-^ 60 MPH (97 km/h)
48 PSI
1
330 kPa
a 20 MPH 32 km/h)
o 40 MPH 64 km/h
* 60 MPH 97 km/h)
48 PSI
I >
330 kPa
FIGURE 5: Dry Traction (Left) and Wet Peak Traction
(Right) Versus Inflation Pressure
18
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THEORETICAL PATH
OF VEHICLE CENTER
OF GRAVITY
FIGURE 6. Geometry of Understeer and Oversteer
Presented below is a summary of three studies which
document the effect of tire inflation pressure on vehicle
understeer and oversteer. The first study is by B. F.
Goodrich Company. This study relates vehicle steering to
a coefficient called the "cornering coefficient." The
other two studies are by the Highway Safety Research
Institute. Both these studies relate vehicle steering to
an understeer/oversteer factor. The important point to
note in reading these summaries is that very large changes
in understeer can adversely effect vehicle handling.
Goodrich Study
A key vehicle parameter that affects the steering
characteristics of a vehicle is the cornering coefficient
of the tires. If a cornering coefficient is too high, the
vehicle will have less understeer and the steering re-
sponse may be too quick for the average driver. A cor-
nering coefficient that is too low will result in insuf-
ficient steering response.
Figure 7 shows the effect of inflation pressure on
cornering coefficient as measured in a study conducted by
by B. F. Goodrich Company. The B. F. Goodrich researchers
who conducted the study point out that coefficients below
those found at 16 psi are unacceptable and coefficients
for radials at 40 and 48 psi are above current practice.
19
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Thus, the results of the B.F. Goodrich study imply that
tires with very low pressure should realize signifi-
cantly improved handling if inflated to manufacturers
recommendations in an inspection program. In addition,
the study also indicates that tires inflated to pressures
ranging between 16 and 40 psi (i.e., the entire target
pressure range that might be used in an inspection
program) have cornering coefficients that are within
current industry practice.
.20
.15
.10
Note break
in scale
A RADIAL
O BIAS-BELTED
DBIAS
16
24
32
40
110
165 220
INFLATION PRESSURE
275
48 PSI
H
330 kPa
FIGURE 7. Cornering Coefficient Versus Inflation Pressure
(100% of Tire and Rim Association Design Load)
(Reference 13)
Highway Safety Research Institute Studies
Researchers at the Highway Safety Research Institute
investigated the effects of tire pressure on steering on a
1971 Mustang.36 The understeer/oversteer factor *(K) was
evaluated experimentally for seven different tire-in-use
conditions. The tire conditions and the corresponding
test results are given in Table 3. As shown, the base
case vehicle has approximately 4.7 degrees per g of under-
steer. This means that for a typical turn with O.lg of
lateral acceleration,* the front wheels are turned .47
degrees more than expected. (Alternately, a 0.47 degree
change in reference steering angle is required to stay on
the same radius path if the vehicle speed is increased
This is the acceleration toward the center of the
turn. The effect on an occupant is often termed cen-
trifugal force.
20
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TABLE 3. Tire Pressure and Understeer
For a 1971 Mustang
Case Tire Configuration
K (+ understeer;
deg/g
1
2
3
4
5
6
Original equipment
24 psi all wheels
Original tires, 24 psi
front, 12 psi rear
Original tires, 24 psi
front, 18 psi rear
Original tires, 18 psi
front, 24 psi rear
Original tires, 12 psi
front, 24 psi rear
Original tires, heavily
Left Turn
4.9
0.9
4.1
5.8
10.4
2.7
Right Turn
4.5
0.9
3.1
7.9
10.6
3.5
worn on front, non-worn
on rear, 24 psi all tires
Original tires on the
rear, radial tires on
the front, 24 psi all
tires
1.0
0.9
Source: Reference 36
enough to obtain a O.lg increase in lateral accelera-
tion) . In cases 2 and 5 where there is a full 12 psi dif-
ferential between the front and rear tires, understeer
changes by as much as 5 deg/g. When the pressure change
is 6 psi (cases 3 and 4), the understeer change is 1 to 2
deg/g.
It should be noted that in case 3, reduced rear tire
pressure of 6 psi causes a decrease in understeer of .8 to
1.4 deg/g whereas in case 4, reduced front tire pressure
of 6 psi causes an increase in understeer of 1.1 to 2.4
deg/g. In most cases if botn the front and rear tires of
a vehicle have their pressure changed by a fixed amount,
the overall effect on vehicle understeer will be less than
21
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if only the pressure of tires on one axle were changed.*
For this reason, several industry spokesmen recommend the
maintenance of front-to-rear tire pressure differentials
when increasing overall tire pressure above manufacturers'
recommendations so that fuel economy is improved with a
minimal effect on vehicle handling.
Calculations presented in another study37 by the
Highway Safety Research Institute give some examples of
vehicle factor changes and the contribution to under-
steer. The study suggests the following for certain tires
and vehicle characteristics:
About 2 deg/g increase in understeer occurs in
changing from power steering to manual steering.
Adding three passengers to the rear seat of a
compact car with E78-14 tires can reduce under-
steer by 1.3 deg/g.
Holding rear tire pressure within +_6 psi will
maintain understeer within +0.5 deg/g at 1,032
pounds load per wheel while at 1,374 pounds load
per wheel the inflation pressure would have to be
held within +_3 psi to keep understeer within +_0.5
deg/g.
The Highway Safety Research institute studies
summarized above indicate that if the pressure of both
tires on a single axle is held within +3 psi of vehicle
manufacturers' specifications, the change in understeer
will generally be under 0.5 deg/g. This change is quite
small; it is about the same as the difference in under-
steer between a left turn and a right turn in the test
vehicle of Table 3 and is substantially less than the
change in understeer that occurs in some vehicles when
adding three passengers to the rear seat. A pressure
change for both tires on a single axle of +_6 psi is likely
to cause an understeer change of 0.5 to 1.5 deg/g depending
on the vehicle. Tnis much change could start to have a
noticeable effect on vehicle operation. Industry spokes-
men have indicated that if the front to rear pressure
differential recommended by manufacturers is maintained as
tires are inflated beyond manufacturers' specifications,
then any effect on vehicle handling will be reduced.
Based on these findings, it can be concluded that:
In one mathematical model of understeer, total vehicle
understeer is due to an understeer factor attributed
to the front wheels minus an understeer factor
attributed to the rear wheels.
22
-------
Inflating very low tires in a vehicle inspection
program will likely improve vehicle handling.
Inflating tires 2 to 3 psi above manufacturers
recommendations will have very little effect on
vehicle handling compared to a vehicle with tires
inflated to manufacturer's specifications.
The effect of pressure increases above manufac-
turers' recommendations will be minimized if
f"ront-to-rear pressure differentials are main-
tained. A pressure change of 6 psi or more on
both tires on only one axle will likely start to
have a noticeable effect on vehicle operation.
Effect of Inflation Pressure on Aligning Torque
Besides changing the understeer on a car, tire pres-
sure changes will also impact the aligning torque of the
front wheels. Aligning torque refers to the tendency of
front tires to straighten themselves during a turn. A
reduction in aligning torque will result in a reduction in
steering feel. As shown in Figure 8. aligning torque
decreases with increasing pressure.38 increasing pres-
sure will reduce steering feel and also adversely affect a
vehicle's natural tendancy to track in a straight line.
.040
S.035
§.030
g
a
z
§.025
.020
A RADIAL
O BIAS-BELTED
D BIAS
16
24
32
40
48 PSI
110
165 220
INFLATION PRESSURE
275
330 kPa
FIGURE 8. Aligning torque versus inflation
pressure (100% of Tire and Rim
Association design load; 1° slip
angle) (Reference 13)
23
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According to Goodrich researchers^ , the loss of
aligning torque is further impacted by the gain in cor-
nering coefficient as pressure is increased. Because of
the increased cornering power, less steer angle is
required and this further reduces the steering feel when
pressure is increased.
Given the above findings, it is difficult to assess
the effect of a change in aligning torque as a result of a
specified pressure change due to the interaction between
aligning torque and cornering on steering feel. Never-
theless, the small pressure changes discussed in the last
section (2 to 3 psi) appear to be sufficiently small that
no problem due to a change in steering feel will be
experienced by motorists.
Experience of Certain Fleets Using Elevated Tire Pressures
To increase fuel economy, several fleets throughout
the country have increased the tire pressure of vehicles
in their fleets to the maximum pressure indicated on the
tire sidewall. This pressure is determined according to
the resistance of the tire to bruising and other types of
failure, whereas the vehicle manufacturers' recommended
pressures are generally set lower to improve the ride of
the vehicle.
Two organizations that have used elevated tire pres-
sures in their fleets for many years to improve fuel eco-
nomy are the Tennessee Valley Authority and the U.S. Pos-
tal Service. Neither of these fleets have reported any
handling problems with the vehicles changed in this way.*
Any changes in vehicle handling that may have resulted
from the pressure changes are not considered a problem by
vehicle drivers.
Summary of Effects of Tire Inflation on Vehicle Handling
The above evidence indicates the following:
Inflation of a vehicles' tires when they are
seriously underinflated is likely to improve the
steering response of the vehicle, and this may be
considered one benefit of a tire inflation
program.
The Postal Service track tests its vehicles before
specifying a pressure setting to insure optimum
economy and vehicle handling.
24
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Tires which are slightly underinflated or over-
inflated relative to the vehicle manufacturers
recommendations (i.e., within 2 or 3 psi) have
only a minimal effect on steering response.
At some point, which may vary from car to car,
overinflated tires (relative to the vehicle
manufacturers recommended specification) may
cause changes in vehicle handling which will be
unacceptable to some motorists. However,
evidence from certain fleets, such as the TVA,
indicates that inflating tires to the maximum
pressure indicated on the sidewall will not cause
unacceptable handling changes.
The maintenance of front to rear tire pressure
differentials will lessen the effect on vehicle
handling of tires inflated to pressures above
those recommended by manufacturers. A 6 psi or
more pressure change on both tires on one axle
will likely start to have a noticeable effect on
vehicle operation.
Thus, in summary, tire inflation programs that are set
to inflate tires above vehicle manufacturers recommenda-
tions will generally not cause unacceptable vehicle
handling effects. Tire and vehicle manufacturers have,
however, shown restraint in recommending elevated tire
pressures, especially more than 3 psi over manufacturers'
recommendations on each axle. Auto manufacturers' recom-
mendations for tire pressure are based upon research which
balances ride, handling, fuel economy and other vehicle-
related factors for a specific auto model. These repre-
sent industry standards that can be used in product
liability litigationhence, manufacturers may hesitate to
recommend overinflation to improve one factor, such as
fuel economy.
TIRE PRESSURE AND TIRE FAILURE
In addition to affecting vehicle handling, the pres-
sure of a tire also has an effect on its resistance to
failure. A grossly underinflated tire can lead to tire
failure from excessive flexing and tire heat buildup-^,
while an overinflated tire is more vulnerable to impact
damage and weakening of the tire body. The effects of
elevated tire pressure on tire failure are best documented
in a recent study conducted by B. F. Goodrich Company.
25
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Researchers at B. F. Goodrich Company recently con-
ducted dynamic bruise tests of tires at different pres-
sures. ' The results of these test are shown in Figure
9. The results are based on tests where researchers mea-
sured the height at which a nub at a 45 degree angle would
puncture a fully loaded tire at 60 miles per hour. As
shown in the figure, tires of all three types of construe- ,
tion are more easily bruised at higher pressures, although
radials and bias-belted tires have a greater bruise resis-
tance than the bias tires. Thus, from the point of view
of a testing program, care should be taken that the
maximum pressures indicated on a tire's sidewall are not
exceeded when the tire is cold (ambient temperature).
§o
z
15 H
12-
9-
6-
3-
0-M
-6
-5
-3
-2
BIAS
32
110
24
165 220
INFLATION PRESSURE
40
4-
275
48 PSI
-I
330 kPa
FIGURE 9. Dynamic Bruise Protrusion Height Versus
Inflation Pressure (100% of T&RA Design Load; 60 m.p.h.)
(Reference 13)
TIRE PRESSURE EFFECTS ON THE VEHICLE SUSPENSION SYSTEM
Only a limited amount of research has been done on the
effects of elevated tire pressures on the structural parts
of the automobile, especially the suspension system.
According to GM engineers, increasing tire pressure 4 to 6
psi can substantially increase loads on tie rods, steer-
ing gears, ball joints and parts of the body and frame.
26
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This may lead to problems in older vehicles that were not
designed for these loads, such as premature wear of
parts. The limited research that has been done suggests
that if pressures are increased more than six psi above
manufacturers' specifications, motorists should be advised
to be sure to include underbody inspection as part of the
regular preventive maintenance for their vehicle.
27
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IV. INCORPORATING A TIRE INFLATION CHECK IN EXISTING
AND PLANNED INSPECTION PROGRAMS
-------
IV. INCORPORATING A TIRE INFLATION CHECK IN EXISTING
AND PLANNED INSPECTION PROGRAMS
The previous chapters have indicated that proper tire
inflation is beneficial both from the standpoint of the
State as well as the motorist. The purpose of this
chapter is to (1) examine alternative tire inflation stra-
tegies, and (2) examine the impacts of incorporating a
tire inflation check in existing and planned emissions,
safety, and combined safety and emissions inspection
programs. The impacts discussed include equipment
requirements, manpower requirements and costs.
ALTERNATIVE INFLATION STRATEGIES
A key policy to be decided when adding an inflation
check to a State motor vehicle inspection program is the
appropriate pressure level for tire inflation. Most
vehicles manufactured in the last ten years have had
recommended inflation pressures between 22 psi and 32
psi. Examples of exceptions to this are indicated in
Table 6. 9 Note that to improve fuel economy
manufacturers have increased recommended pressures
significantly for recent car models. Many vehicles also
have a difference in pressure specified between the front
and rear tires. This differential is usually under 4 psi,
but for some models, especially pre-1978 Volkswagens,
Corvettes and station waqons the difference can be much higher,
This section describes three different methods or
strategies of inflating tires in a state motor vehicle
inspection program. The three strategies are:
. . Inflating all tires to the manufacturer's
recommended pressure, adjusted for tire
temperature.
Inflating all tires to 3 psi above the
manufacturer's recommended pressure, adjusted for
tire temperature, witn the provision that no tire
be inflated above the maximum pressure on the
tire sidewall when adjusted for temperature.
Inflate all tires to the maximum sidewall pressure,
usually 32 psi, adjusted for temperature.
Although other specific inflation strategies may be chosen
by certain states, .these three strategies represent the
range of choices available to states. The benefits
28
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TABLE 6. Examples of Vehicles With Manufacturers
Recommended Pressure Less Than 22 PSI or More Than
32 PSI on Models Manufactured Since 1972
Pressure
Vehicle Front Rear
Volkswagen Beetle, Wagon,
Convertible, Karmann Ghia 17-18 26-29
1953-1977
Porsche 8-928 1978-1981 36 36
Peugeot 72-79 Station Wagon 23 40
Mercedes 4-190, 1960-68 21 27
Pontiac, LeMans 6 cylinder
1978-1979
Corvette 1973-1977
Matador Station Wagons 1972-1978
1981: Chevrolet Malibu
Oldsmobile Cutlass
Pont iac , LeMan s
Plymouth TC3, Horizon,
Reliant
Buick Century
Ford Escort
Ford Mustang
28
20
20
35
35
35
35
35
35
35
35
26
28
35
35
35
35
35
35
35
Source: Reference 29
associated with each of these strategies are shown in
Table 7. Details are provided in Appendix A. As can be
seen in the table, considerable extra benefits are gained
in Strategies 2 and 3 which involve pressures greater than
the manufacturers' recommended specifications. The
operational requirements and safety considerations
associated with each of these inflation strategies are
discussed below.
29
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TABLE 7. Summary of Benefits by Inflation
Strategy (Dollars Per Vehicle)
INFLATION STRATEGY
Inflate to 3
Inflate to Vehicle psi Above
Manufacturers' Manufacturers' Inflate to Maximum
Benefit Recommendations Recommendations Sidewall Pressure
Fuel Economy Savings $3.59 $8.47 $13.94
Due Directly to Program
Treadware Savings 1.23 2.82 4.54
Due Directly Savings
Fuel Economy and Treadware
Savings Due to Improved
Consumer Maintenance 6.07 8.89 10.24
Strategy 1; inflate All Tires to the Manufacturer's
Recommended Pressures
For this strategy, an inspector in an inspection/
maintenance program would determine the manufacturers'
recommended pressures of each vehicle and inflate the
tires to the specified pressures. The auto .manufacturers'
recommended pressures are easily accessible on most
vehicles manufactured since the late 1960's and are most
often found on the door pillar of the driver's side.
Reading the label will normally take about 10 seconds.
Pressures for vehicles without information placards on the
vehicles can readily be obtained from tire booklets, such
as the "Tire Guide."29
Once the inspector has determined the manufacturer's
recommended tire pressures, the next step is to inflate
the tires. Even with the correct vehicle manufacturer's
recommended pressures known, however, it is still not
clear what pressure should be set for each tire since the
tire temperature is not known. According to the Rubber
Manufacturers Association a typical tire can increase in
pressure up to 4 psi when hot (as when driven for a half
30
-------
hour at highway speeds). Manufacturers' recommended
pressures are specifically set for a cold tire, one driven
less than a mile. At a State inspection facility, some
tires will be cold and many will be hot. The Tire
Industry Safety Council recommends that if a tire is
inflated when hot, the recommended pressure should be
increased 3 to 4 psi and then rechecked when the tire is
cool. This would be a reasonable strategy for a State
inspection program given a procedure for determining a
"hot" tire. A simple and commonly employed method of
determining a "hot" tire is by placing one's hand on the
sidewall of the tire. A "hot" tire will be noticeably
warm to the touch. While somewhat crude, this procedure
is simple and can be easily incorporated in a state
inspection program. It may be possible to develop a
method to measure tire sidewall temperature to more
accurately determine correct tire pressure. However, no
such procedure has apparently been developed to date and
the operating inconvenience seems unwarranted given the
small potential error in setting pressure by the more
approximate means described above.
Inflation strategy 1 for a State inspection program
can be summarized as follows:
Read the manufacturers recommended pressure off
the information placard on the vehicle.
Inflate underinflated tires to the manufacturer's
recommended pressure. For "hot" tires increase
the pressure by 3 to 4 psi.
Inform the motorist of the importance and respon-
sibility of maintaining proper tire pressure (See
Chapter V) .
Inflation strategy 1 provides the least benefits of the
three strategies outlined in this section. It also
represents the most conservative approach to tire
inflation since the manufacturers recommended inflation
pressures are strictly followed.
Strategy 2; Inflate All Tires to 3 psi Above the
Manufacturers' Recommended Pressures
For strategy 2, tires are inflated to 3 psi above the
vehicle manufacturer's specifications. As described in
Chapter II, increased tire pressure beyond that
recommended by the vehicle manufacturer will continue to
improve fuel economy and treadwear. The Tire Industry
Safety Council Recommends: "For increased gas mileage and
optimum tire performance add 2 to 3 psi more than
31
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indicated on the information placard. However, never
exceed the maximum inflation pressure as indicated on .the
tire sidewall." Further, as discussed in Chapter II, a
pressure change of 2 to 3 psi will have only a minimal
impact on vehicle handling and safety, especially since
the front to rear pressure differential is maintained.
The additional savings for the motorist are significant,
as indicated in Table 7. As with strategy 1, an
additional 4 psi should be added to the tires if they are
hot.
Strategy 2 can be summarized as follows:
Read the manufacturer's recommended pressure off
the information placard on the vehicle. Note
that the front and rear tires have different
recommended pressures.
Add 3 psi to the recommended pressure. Make sure
this is not higher than the maximum pressure on
the tire sidewall.
For hot tires add another 3 to 4 psi to all tires.
Inform the motorist of the importance and
responsibility of maintaining proper tire
pressure (See Chapter V).
Strategy 3: Inflate All Tires To The Maximum Sidewall
Pressure
For strategy 3, all tires are inflated to the maximum
pressure indicated on the tire sidewall. This pressure is
usually 32 psi although for many newer tires it is 35
psi. By inflating all tires to the maximum tire sidewall
pressure vehicle inspectors will not have to identify the
manufacturers' recommended pressures. This can save time
and also eliminate the inconvenience caused by attempting
to identfy the recommended pressure when the information
placard is not present or is obscured. However, since
only a single pressure is used for a wide variety of
vehicles, the pressure set compared to the vehicle
manufacturers' recommendations will have a wide range.
Based on the discussion of safety and handling effects
of tire pressure in Chapter III, it would be prudent to
avoid inflating the tires on certain vehicles with unusual
manufacturers' recommended pressure settings. As a
general guide, vehicles with 6 or more psi front to rear
pressure differentials should not be inflated in the
program. This would eliminate such vehicles as station
wagons, Corvettes and Volkswagen Beetles. A specific list
32
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of excluded vehicles can be assembled before program
start-up by using a source such as the "Tire Guide"
referenced at the end of this report.29 Assembling such
a list at the beginning of a program will save time during
the inspection process, since the inspector will not have
to look up the vehicle manufacturers recommended pressures
do identify "exempt" vehicles.
Strategy 3 can be summarized as follows:
Make sure the vehicle is not on the list of makes
to be excluded from the inflation program
Identify the maximum pressure indicated on the
sidewall and inflate all tires to this pressure.
If the tire is hot, add 3 to 4 psi.
With strategy 3, there would be little variance in
operating procedure from car to car. The benefits of
strategy 3 as shown in Table 7 are more than for strategy
2. While this strategy may change the handling
characteristics of certain vehicles, experience of fleets
such as the TVA indicate that this will be acceptable to
most drivers.
EQUIPMENT REQUIREMENTS
Since most decentralized inspection facilities would
already have tire inflation equipment the following
discussion pertains solely to centralized inspection
programs. Two basic pieces of equipment are needed to
inflate tires in a centralized inspection program: a
compressor and a tire gauge. While the number of gauges
per facility would be proportioned to the number of lanes,
for an inspection facility with as many as four lanes,
only a small air compressor would be required to service
all the lanes. Examples of such units with prices are
shown in Table 8. A typical air compressor (see Figure
10) requires 4 1/2 feet by 2 1/2 feet of floor space and
3 1/2 to 4 feet of height. The compressors generally
provide air at pressures considerably higher than that
needed to inflate tires, usually over 100 pounds per
square inch. As a result, the compressor could be used
for other purposes as well, including powering lifts and
tools.
33
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TABLE 8. Tire Inflation Equipment
Type of
Equipment
Air Compressor
Tire Inflator
with Integral
Pressure Gauge
Equipment
Description
Compressor & Tank
Compressor & Tank
Compressor & Tank
Tankless compressor
Self-service
compressor
In-line gauge
Preset tower gauge
Typical
Manufacturer
Ingersoll Rand
Model 2-253.35
Dresser -Wayne
10 HP Deluxe
Ingersoll Rand
5 HP
ECO 108C
Champion
3/4 HP
Grover Model -
907
ECO 97
Price
$4200*
$5000*
$1400*
$1325
$ 350*
$31.00
$445.00
Any piping will be extra
In order to inflate tires to the variety of pressures
now specified by manufacturers, a means of measuring and
adjusting pressure is required. Two methods are feasi-
ble. One uses a gauge in-line with the tire pressure hose
as shown in Figure 11. When the hose is connected to the
tire the gauge reads the tire pressure. The inspector can
insert air into the tire by depressing a lever on the
gauge and upon releasing the lever the pressure can again
be read. In this manner a tire can be quickly and accu-
rately (within 1 pound per square inch) inflated. Alter-
nately a gauge can be purchased such as the one shown in
Figure 12 where the pressure is set to a specified amount
and the operator merely holds the hose to the tire.
Prices for typical gauges are also shown in Table 5.
34
-------
FIGURE 10: A Typical 5 HP Compressor
FIGURE 11: An In-Line Gauge
35
-------
±
FIGURE 12: An Air Tower With Pre-Set Pressure
Certain other types of inflation equipment have yet to
be developed but may be helpful in the future. Operators
of centralized programs should discuss these with their
contractors or equipment vendors as appropriate to see if
they may be feasible. Ideas include:
A computer library of vehicle tire
specifications, indexed by make, model year, etc.
A method of measuring the pressure automatically
with a device that mechanically squeezes the tire
sidewalls. Research has been made along these
lines, but to date such equipment is not
available.
The incorpration of automatic tire temperature
measurement and pressure adjustment in the
inflation equipment.
An improvement to the air tower (Figure 12) that
would allow pressure adjustment at the end of the
hose and would increase the speed of inflating
tires.
36
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MANPOWER REQUIREMENTS AND COST IMPACTS
No existing state inspection program inflates tires as
part of the regular inspection process. However, the Key-
stone Auto Club (AAA) in Philadelphia inflates tires on a
regular basis in its diagnostic center. In this center
tire inflation is checked while vehicles are in a ground-
level station (as opposed to up on a lift) . The inspector
opens the driver's door and reads the recommended tire
pressure on the tire inflation placard. He then uses a
hose with an in-line gauge to inflate the tires. As shown
in Table 9, the process consistently takes about a minute
and forty seconds.
TABLE 9. Approximate Times to Inflate Tires
Procedure Time
Read pressure on door 10 sec.
Pressurize tire one 20 sec.
Pressurize tire two 20 sec.
Walk around car 10 sec.
Pressurize tire three 20 sec.
Pressurize tire four 20 sec.
TOTAL 1 min. 40 sec,
Source: Time studies made by Booz, Allen and Hamilton at
the Keystone Auto Club
Given that the times presented in Table 6 are repre-
sentative,* the remainder of this section discusses the
manpower requirements and cost impacts of incorporating a
tire inflation check into existing centralized and decen-
tralized inspection programs. The section is divided into
the following parts:
Checking the Tire Pressure and Inflating the
Tires If Necessary in Centralized Inspection
Programs.
The times are for an in-line tire gauge. Use of the
tower-type gauge would probably result in longer times
if different front and rear tire pressures required
resetting the pressure.
37
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Checking the Tire Pressure and Inflating the
Tires If Necessary in Decentralized Inspection
Programs.
Checking Tire Pressure Only in Centralized and
Decentralized Inspection Programs.
Checking Tire Pressure and Inflating Tires if Necessary in
Centralized Inspection Programs
This section presents four examples of an inflation
check incorporated into existing centralized inspection
lanes. Each example consists of a schematic diagram show-
ing the existing layout of the lane followed by a discus-
sion of the best location for the inflation check plus the
manpower requirements and costs associated with adding the
check to the lane. The purpose of these examples is to
demonstrate the feasibility of the check and to provide
guidance to program planners on the best location and cost
impact of the check.
A central criterion used to determine the best loca-
tion for the inflation check in the examples which follow
was that it have minimal impact on the throughput of the
lane.* As a general rule, it is only feasible to add
equipment and or tests to existing facilities if they have
little or no impact on throuhput. For those planners now
considering centralized inspection programs, it should be
recognized that you now have an opportunity to incor-
porate a tire inflation check into the program at low
cost. Failure to think ahead can result in the need for
costly facility expansion at a later date.
Given this criterion, in determining the best location
for the inflation check the following general rules were
followed:
Whenever possible personnel were added to keep
throughput constant rather than maintaining the
existing personnel level and decreasing through-
put. In general, it is less expensive, at least
in the short term, to handle an increased inspec-
tion load by adding personnel than to allow
throughput to drop to the point that new facili-
ties are required.
Throughput is the number of vehicles that can be pro-
cessed in one lane or facility over a specific period
of time and is commonly expressed in vehicles per lane
per hour. A significant drop in a facility's through-
put translates into longer hours of operation or the
necessity for the construction of new facilities to
handle unmet processing requirements.
38
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Maximum use was always made of the existing
inspectors' time. Making use of this time helps
to offset the costs of adding inspectors to keep
throughput constant.
When at all possible, the check was added to that
station with the greatest amount of slack time.
This helps to balance the lane. A well balanced
lane is most efficient.
Because it is likely that your program will differ from
the examples provided, you should also follow these same
rules in determining the best location for the check in
your program.
In determining the cost impact of adding the check to
the lane, the following was assumed:
An inspector salary including fringe of $18,000.
Straight line depreciation of equipment over five
years at an annual interest rate of 12 percent.
Distribution of all additional equipment, person-
nel and other costs over all vehicles in the
program.
A detailed methodology for estimating costs is presented
in Appendix B. Again because it is likely that your pro-
gram will differ from the examples provided, it is recom-
mended that you use the methodology contained in
Appendix B to determine the exact cost impact of adding
the inflation check to your program.
Example No. 1: Delaware
This is an example of a centralized safety-only
inspection program with a platform brake tester. It is
typical of centralized safety inspection programs
currently planning the addition of an emissions test to
their program. As shown in Figure 13, the lane layout of
these facilities consists of three stations as follows:
The first station is vehicle check-in and wheel
alignment. This is where the inspector checks
the motorist's registration certificate against
the vehicle license plate, prepares an inspection
card, and checks the vehicle's horn, windshield
wipers and lights for proper operation. Vehicle
front end alignment is also checked at this sta-
tion using a scuff gauge, and all the glass areas
on the vehicle are checked for cracks.
39
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STATION 3
PLATFORM
BRAKE TESTER
AND CHECKOUT
(39 seconds)
STATION 2
OPTICAL
HEADLIGHT AIMER
140 snondsl
STATION I
CHECK IN
AND SCUFF GUAGE
169 seconds)
FIGURE 13. Delaware Centralized Safety Lane
At the second station headlamp aim is checked
using a track mounted optical headlamp aimer.
Vehicle mileage is also checked at this station
and recorded on the inspection card.
At the third and final station, the vehicle's
service brakes are tested, and the results of the
inspection are provided to the motorist. In
testing the vehicle's service brakes, a platform
brake tester is employed.
The inspection employs three inspectors and in total
takes slightly over 3 minutes. Maximum lane throughput is
approximately 52 cars per hour (60 minutes divided by the
time at the first Station --1.15 minutes). Delaware
motorists presently do not pay a separate inspection fee.
Rather the cost of inspection is included in their annual
registration renewal fee.
Other jurisdictions with centralized safety-only
inspection programs include the City of Chatanooga,
Memphis, New Orleans, Florida's Orange, Duvall, Broward
and Dade counties, and the District of Columbia. Of these
40
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jurisdictions, Dade County, Florida and the District of
Columbia have roller brake testers in lieu of platform
brake testers. Other than this difference, the
characteristics of these programs are similar to those
just described for Delaware.
The best location for the tire pressure check in the
Delaware lane would be Station 1, ideally prior to the
vehicle running over the scuff gauge. Inflation at this
point would not interfere with any other operation and the
vehicle would be located there longer than at any other
station. In addition, proper tire inflation could improve
the efficiency of the brake test at Station 3. However,
throughput would be significantly reduced if only one
additional inspector was added. Thus, tne only feasible
option for a Delaware type lane with similar tests would
be to inflate the tires using two inspectors (one on each
side of the vehicle). By adding two additional inspectors
to Station 1, tire pressure can be checked on all four
wheels in about one minute. The incremental cost of
performing this test if distributed over all vehicles
would be about $.89.
If Delaware converts its safety lane to a safety-
emissions lane by adding emissions inspection at Station
2, then tire inflation could still take place at Station 1
without difficulty. Adding a tailpipe emissions test to
Station 2 would require one additional inspector and would
cost an additional $0.52.*
Example No. 2: Arizona
This is an example of an emissions-only centralized
inspection program. With the exception of Oregon, it is
typical of other existing and planned emissions-only in-
spection programs, although unlike Arizona most of these
other programs will not have dynamometers. As shown in
Figure 14, each lane consists of three stations as fol-
lows:
Vehicle Check-in (Station 1). At this station,
the inspector obtains vehicle registration infor-
mation from the motorist and keys this infor-
mation into the CRT unit. The inspector also
collects the inspection fee and completed repair
form in the case of a reinspection.
U.S. Department of Transportation, An Evaluation of
Existing Motor Vehicle Diagnostic Inspection Concepts,
Volume V, prepared by Booz, Allen and Hamilton, June
1981, Contract No. DTN H22-80-C-05018.
41
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STATION 3
DATA PRINTER
1601.
RPM INDICATOR
STATION 2
EMISSIONS ANALYZER
EXHAUST LOUVERS
STATION 1
DATA ENTRY TERMINAL
FIGURE 14. Arizona Emissions Inspection Lane
Emissions Test (Station 2). At this station, the
vehicle undergoes an emissions test. The test
consists of sampling the exhaust gases of a
vehicle using an infrared emissions analyzer
while the vehicle is idling. However, if the
vehicle fails this test, it is then precon-
ditioned at 30 miles per hour while loaded on a
dynamometer and retested at idle. Programs other
than Arizona will accelerate the engine to 2500
rpm and then retest at idle. This avoids the
need for a dynamometer.
Vehicle Check-out (Station 3). At this station,
the inspection results are printed out and a copy
is given to the motorist.
42
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The total inspection involves three inspectors (one per
station) and takes approximately 5 minutes per vehicle.
The maximum throughput of the lane is 27 vehicles per
hour, and the cost per vehicle is $5.44.
Like Delaware the best location for the tire inflation
check given this type of facility layout is Station 1.*
Inflation at this station would not interfere with other
operations. One additional inspector would be required,
and there would be no impact on throughput. The
incremental cost per vehicle would be the additional
inspector's salary plus the annualized costs of the tire
inflation equipment** -- approximately $0.71.
Example No. 3: New Jersey
This is an example of a typical centralized safety
plus emissions program operated by State personnel. A
schematic diagram depicting the layout of a typical lane
in a New Jersey facility is shown in Figure 15. As shown,
a typical lane consists of five stations as follows:
Station 1; Vehicle Check-in and Emissions Test.
At this station the inspector checks the vehicle
registration, fills out the inspection card and
performs an idle emissions test for HC/CO.
Station 2; Scuff Test. At this station the
vehicle's front end alignment is checked using a
scuff gauge. In addition, the vehicle's wind-
shield wipers, glazing, stop lights and turn
signals are checked.
Station 3; Front End Lift. This station
involves a test of the vehicle's steering and
suspension system. Steering lash and play are
also checked.
Station 4; Headlamp Aim. This station involves
a test of the vehicle headlamp aim using a track
mounted optical headlamp tester. Both left and
right, high and low beams are tested.
* It could also be added to Station 3 without any impact
on throughput, however it is best to add it to Station
1, especially for those states planning loaded mode
testing due to the safety considerations associated
with using a dynamometer.
** Arizona stations already have compressed air equipment
to operate the dynamometers. This would reduce the
capital requirements.
43
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STATIONS
PLATFORM
BRAKE TESTER
AND CHECKOUT
STATION 4
OPTICAL
HEADLIGHT AIMER
STATION 3
FRONT END LIFT
STATION t
SCUFF GUAGE
STATION I
CHECK IN
AND EMISSIONS ANALVZER
FIGURE 15. New Jersey Safety and Emissions Lane
Station 5; Brake Test and Vehicle Checkout. At
this final station, the vehicle undergoes a plat-
form brake test. In addition, the parking brake
and brake pedal reserve are also checked. Upon
completion of these tests, the motorist is given
the results of the inspection, and a sticker
indicating the results is affixed to the
windshield.
The total inspection as described above involves 5 inspec-
tors and takes about 5 minutes per vehicle. The maximum
throughput of each lane is about 36 vehicles per hour.
The cost of the inspection is set by statute and is cur-
rently $2 per vehicle.
Again the best location for the tire inflation test
(as in the other two examples) is Station 1, although the
test can also be easily accomodated at Station 4 without
any impact on throughput. Station 1 is recommended
because it increases the accuracy of the scuff and plat-
form brake tests (Stations 2 and 5) .
The test would require an additional inspector and
would cost about $0.71 per vehicle.
44
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Example No. 4: Washington State
Washington state is planning to operate an
emissions-only inspection lane with only one station and
inspector at each lane. At this station a fee will be
collected and the vehicle registration number input into a
computer. The emissions will then be checked. The whole
procedure should take about two minutes.
Adding tire inflation to a lane of this type would
require one additional inspector working simultaneously
with the emissions inspector. Since the test time at the
facility is two minutes, the cost of the additional
inspector will be about the same as in example number 2,
$.71.
Checking Tire Pressure and Inflating Tires if Necessary in
Decentralized Programs
Unlike centralized inspection facilities, maintaining
a constant throughput, is not a central criterion for the
addition of a tire inflation check to decentralized
facilities. In most decentralized inspection facilities,
the inspection is performed by one person. Depending on
the type of inspection and the items inspected, the time
ranges from 5 to 45 minutes, and the cost per vehicle
ranges from $2 to $15.
In general, tire inflation in decentralized programs
should pose no particular difficulty. Most facilities
that can perform an emissions or safety inspection nor-
mally would have the compressors and gauges necessary to
inflate tires. The time requirement to inflate the tires,
however, may be somewhat longer than for centralized
facilities due to the inefficiency of decentralized facil-
ities compared to centralized facilities, but a time of
two minutes would be reasonable.
Thus, given the following assumptions, a tire infla-
tion check could be added to a decentralized inspection
program for about $1.00:
All inspections would be conducted by the same
person.
Equipment if not already present would be
amortized over a five-year period at an annual
interest rate of 12 percent and would be
completely amortized after five years.
45
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The total cost of performing any additional tests
including the amortized cost of equipment would
be passed onto the consumer.
The average licenced inspection facility would
conduct 1,500 inspections a year, about six per
day, and that the shop hourly service rate would
be $30 per hour.
A detailed methodology for calculating the cost impact of
adding a tire inflation check to a decentralized inspec-
tion facility is contained in Appendix B.
The above assumptions were made to illustrate the
likely cost impact of adding a tire inflation check to a
decentralized inspection facility. Because it is likely
that your program differs from the assumptions we have
made here, it is recommeded that you use the methodology
contained in Appendix B to determine the exact cost impact
of adding a tire inflation test to your program.
Checking Pressure Without Inflating Tires in Centralized
and Decentralized Programs'
Some States may wish to add a less costly tire check
to their programs. One concept that would add only pen-
nies per vehicle to the cost of a program is based on a
simple pressure measurement of one tire. An inspector
with 15 or 20 seconds of slack time could measure the
pressure on one tire and report to the motorist if the
reading was low.
The motorist would be given appropriate consumer
information (see Chapter V) and encouraged to inflate his
or her tires to the proper level at a series of tire pres-
sure towers located conveniently outside the facility or
in a separate bay. While this approach would be less
expensive than actually inflating a vehicle's tires, no
benefits can safely be assumed as a result of the pressure
checks since no inflation takes place. Also, since only
one tire is checked, many vehicles with low tires may not
be detected at all. However, some of the consumer
information benefits indicated in Table 7 may still be
achieved with this type of approach.
LIABILITY ISSUES
Contractors or states operating inspection programs may
be concerned about potential liability claims, both legiti-
mate and frivolous, if their employees inflate tires. Claims
could be associated with tire failure, tire damage or acci-
dents allegedly caused by vehicle handling problems. While
46
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the material outlined in Chapter II and the recommenda-
tions made in this chapter should provide for safe program
operation, the possibility still exists that a particular
vehicle may at some time experience problems which could
be related to the tire inflation. States may consider
several methods to reduce the risk or impact of such
claims:
Obtaining the owner's consent before inflating
tires
Emphasizing in the consumer information
literature that the motorist should check his
vehicle's tire pressure regularly
Not inflating tires when the correct tire
pressure cannot be determined (for strategies 1
and 2) or where there is doubt whether the
vehicle should be included or excluded from the
program (strategy 3) .
Careful training of inspectors
Adequate insurance coverage
Set procedures for processing any claims.
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V. METHODS OF MAINTAINING PRESSURE
AFTER INSPECTION
-------
V. METHODS OF MAINTAINING PRESSURE
AFTER INSPECTION
As discussed in detail in Appendix A, typical tires
lose one-half to one psi per month. A large part of this
pressure loss is due to air permeating through the tire
body. In addition, a vehicle's tire pressure will change
by about 1 psi for every 13 degrees Fahrenheit of tempera-
ture change. Thus, tire pressure should be checked regu-
larly; the Tire Industry Safety Council recommends that
tire pressure be checked at least monthly.
Tire pressure checks incorporated into emissions,
safety or combined safety and emissions inspection pro-
grams will occur once or, in a few cases, twice a year,
not frequently enough to ensure motorists are maintaining
proper tire pressures on their vehicles. Thus methods of
encouraging motorists to maintain proper tire pressure
throughout the remaining part of the year following
inspection are required. This chapter describes such
methods.
LOW TIRE PRESSURE WARNING INDICATORS
One method is through the use of low tire pressure
warning devices. Low tire pressure warning devices have
been on the market for a number of years. These devices
generally are attached to the valve stem of each tire and
provide a visual signal when the tire pressure drops below
a set value. Prices around $1 to $3 per wheel are typi-
cal.* With the use of these trigger mechanisms, the vehi-
cle owner needs to only visually inspect each wheel to see
if the tires are inflated properly.
In August, 1981, Consumer Reports reported on a test
of a low tire pressure warning device made by Brookstone
Company in Peterborough, N.H. The device was tested on
several vehicles for up to 1200 miles of use. The
magazine reported reliable and accurate operation of the
product.
The National Highway Traffic Safety Administration, up
until recently, was considering a proposal which would
Prices would be lower if the devices were purchased in
large quantity for distribution by States. Electronic
warning systems cost substantially more, usually $25
or more per wheel.
48
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require all motor vehicles to have a low tire pressure
warning system.* A review of the comments submitted by
tire manufacturers and tire organizations was made to
obtain insights as to the effectiveness of these de-
vices.22 Based on this review, it was found that
industry experience with on-tire devices is in general
quite negative. For instance, the Rubber Manufacturers
Association stated:
"No low pressure warning device that we know of has
ever met the criteria of high reliability and afford-
able costs necessary to be a useful addition to pas-
senger cars and trucks.2-^"
Rolls-Royce stated:
"To date, systems evaluated by Rolls-Royce have not
proved reliable over long periods of general system
inactivity. This is a worrying feature as the re-
quired provision of a warning system immediately
dilutes the user's responsibility for checking pres-
sures regularly.2'*"
Thus, while tire pressure warning devices can help a
motorist check tire pressure more easily, periodic checks
(one a month) must still be made to insure that the device
is operating properly.
A new trigger device that replaces the regular tire
valve is supposed to be much more reliable than currently
available tire pressure warning devices. (See Figure 16).
It may soon be marketed by Schrader. However, this device
can only be installed upon a tire change.
Given the above findings, State inspection programs,
as a minimum, should inform motorists that the use of low
tire pressure warning devices do not preclude them from
periodically checking tire pressure. The devices may make
pressure monitoring easier, but they do not eliminate the
need for all precautionary checks. If new devices, such
as the valve-replacement system, become available and are
shown to be reliable, these can be recommended as replace-
ment valves when new tires are purchased. Even then,
motorists should still be encouraged to check tire pres-
sure at least once a month, especially if the automatic
devices are set to trigger at several psi below the manu-
facturer's recommended pressure.
The proposal was dropped by the Reagan Administration.
49
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FIGURE 16.
Low Tire Pressure Warning Device That
Replaces The Tire Valve
CONSUMER INFORMATION
Another method of encouraging motorists to periodi-
cally check tire pressure is through the distribution of
consumer information on tire maintenance. A consumer
information program could easily be incorporated in. a
State motor vehicle inspection program. For example, a
consumer information program could take either of the fol-
lowing two approaches:
It could take the form of booklets or pamphlets
that are distributed to motorists either upon
entering or leaving the inspection facility.
It could take the form of wall charts that the
motorists could read while waiting for the
inspection.
A number of booklets oriented towards consumers on tire
maintenance are available from the Tire Industry Safety
Council in Washington, D.C. free of charge.*
A modest fee would most likely be charged if large
quantities of booklets were ordered.
50
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Regardless of the approach taken, the following is a
list of some of the major points that should be stressed
in any information program on tire maintenance that is
oriented toward consumers:
Underinflation has several detrimental effects.
These include:
Reduced tire life
Reduced fuel economy
Increased chance of blow outs
Proper inflation can be found on the tire sticker
in the car or the owner's manual.
An accurate tire pressure gauge should be pur-
chased. These cost only $2 to $3.*
Tire pressure should be checked at least monthly
and before long trips.25
When tire pressure is checked, make sure tires
are "cold" or have been driven less than a mile.
Pressures typically rise three or four pounds per
square inch (psi) when hot. If it is necessary
to add air to a hot tire, inflate it three or
four psi above the recommended pressure. The
tire should be rechecked when cool and readjusted
if necessary.26
Tire pressures can generally be increased 2 to 3
pounds to improve fuel economy and tread life
without significantly affecting vehicle handling.
Higher pressures will further improve fuel eco-
nomy and tire tread life, but the motorist should
do this with caution since the handling charac-
teristics and ride of the vehicle may change. A
motorist wishing maximum economy could inflate
all tires to the maximum sidewall pressure
allowed realizing that a slight adjustment to
changed handling may be required. If front to
rear pressure differentials specified by the
vehicle manufacturer are maintained any changes
in handling will be reduced. Do not exceed the
limit molded on the tire sidewall when the tire
is cold (this limit however can be exceeded by 3
to 4 psi when the tire is hot).
Prices would be lower if a state purchased large
quantities of gauges and resold them to the public.
Since some cheap gauges are inaccurate) a state may
want to recommend certain gauges it knows are accurate,
(See Consumer Reports, February, 1980).
51
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Make sure all tire valves and extensions where
possible are equipped with valve caps to keep out
dirt and moisture. Installing a new valve
assembly is good insurance whenever a tire is
replaced. ' New valve assemblies with a
special trigger to alert the driver when tire
pressure is low can be installed and should help
improve tire maintenance.
AVAILABILITY OF COMPRESSED AIR FOR PUBLIC USE
In recent years there has been growing concern over
the availability of compressed air for public use. Many
service stations have switched to self-service operation
and have eliminated air towers or hoses for use by the
public. Table 10 shows the number of motorist service
facilities of different types where motorists should be
able to inflate tires, according to a major manufacturer
of air towers.28 According to this manufacturer, nearly
one-half of the service facilities do not have compressed
air with gauges or a tire inflation capability. Further
investigation by the Department of Energy indicates that a
considerably smaller number actually have accessible,
working air towers for the public. Many towers are broken
or inaccurate due to poor maintenance by station owners.
TABLE 10. Vehicle Service Facilities
Number of
Facility Type Facilities
Full Service 76,700
Split Island 38,600
Self-Service 24,200
Convenience 13,100
Mini-Service 5,200
Truck Stop 2,400
Car Wash 2,400
Car Care 9,600
TOTAL 172,200
Source: Reference 28.
Recently, a number of facilities that normally would
not have had the large compressors required in service
station operation (for lifts and power tools) have been
52
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able to offer compressed air to the public by purchasing
relatively small, inexpensive compressors attached to coin
operated pressure towers. The increase in these systems,
however, is only slightly mitigating the general problem
of compressed air availability.
Thus, given the declining number of service stations
with air, other useful services an inspection facility
could perform to encourage the public to maintain the pro-
per tire pressure on their vehicles include the following:
Provide information on the location of tire
pressure towers convenient to the motorists'
residences
Provide tire pressure towers at inspection
facilities
Encourage the installation or repair of tire
pressure towers at other facilities.
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REFERENCES
1. Owen J. Viergutz, Harold G. Wakeley and Larry Dowers,
"Automobile In-Use Tire Inflation Survey," Society of
Automotive Engineers, Inc., Warrendale, Pennsylvania
15096, Number 780256, February 1978.
2. T. Taylor and S. K. Clark, "Effect of Automobile Tire
Pressure Changes on Fuel Economy," DOT-HS-9-02110,
Corporate-Tech Planning, Inc., Waltham, Massachusetts,
March 1980.
3. Taylor and Clark, "Effect of Automobile Tire Pressure."
4. L. Forrest, W. B. Lee, and W. M. Smalley, "Evaluation
of Techniques for Reducing In-Use Automotive Fuel
Consumption," The Aerospace Corporation, El Segundo,
California, Number ATR-78(3851)-IND, June 1978.
5. Glenn D. Thompson and Martin E. Reineman, "Tire
Rolling Resistance and Vehicle Fuel Consumption,"
Society of Automotive Engineers, Number 810168,
February 1981.
6. Bruce C. Grugett, Martin E. Reineman and Glen D.
Thompson, "The Effects of Tire Inflation Pressure on
Passenger Car Fuel Consumption," Society of Automotive
Engineers, Number 810069, February 1981.
7. D. A. Clemming and P. A. Bowers, "Tire Testing for
Rolling Resistance and Fuel Economy," Society of
Automotive Engineers, Number 750457, February 1975.
8. L. Forrest, "Evaluation," p. 3-45.
9. Taylor and Clark, "Tire Pressure Changes," p. 3-4.
10. Grugett, et al., "Tire Inflation Pressure," p. 3-7.
11. "Preliminary Regulatory Analysis for Low Tire Pressure
Warning Indicator," National Highway Traffic Safety
Adminstration, Office of Plans and Programs, October
1980.
12. Bruce Grugett, "The Effect of Tire Inflation Pressure
on Vehicle Fuel Economy," EPA-AA-SDSB-80-04, April
1980.
54
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13. B. L. Collier and J. T. Warchol, "The Effect of
Inflation Pressure on Bias, Bias-Belted and Radial
Tire Performance," Society of Automotive Engineers,
Number 800087, February 1980.
14. P. S. Francher et al., "Steering Controllability
Characteristics," DOT HS-6-01409, Highway Safety
Research Institute, Ann Arbor, Michigan, August 1977,
Chapter 3.
15. Collier, "Effect of Inflation Pressure."
16. Paul S. Fancher, James E Bernard and Lloyd H. Emery,
"The Effects of Tire-in-Use Factors on Passenger Car
Performance," Society of Automotive Engineers 741107,
October 1974.
17. Collier, "Effect of Inflation Pressure."
18. "Consumer Tire Guide," Tire Industry Safety Council,
Washington, D. C.
19. Robert E. Scott, Charles P. Compton, Lyle D. Filkins,
"Vehicle Handling Study, Second Interim Report,"
UM-HSRI-77-44. The University of Michigan, Highway
Safety Research Institute, December 1977.
20. "Tri-Level Study of the Causes of Traffic Accidents,"
DOT HS-801 334 and DOT HS-801 335, Institute for
Research in Public Safety, Indiana University, Vol. I
pp. 4453 and Vol. II, p. F-50.
21. "Preliminary Regulatory Analysis for Low Tire Pressure
Warning Indicator," National Highway Traffic Safety
Administration, Office of Plans and Programs.
22. National Highway Traffic Safety Administration, Docket
81.05.ANPRM.N01.
23. National Highway Traffic Safety Administration, Docket
81.05.ANPRM.N01.068.
24. National Highway Traffic Safety Administration, Docket
81.05.ANPRM.NO1.128.
25. "Consumer Tire Guide," Tire Industry Safety Council.
26. Tire Industry Safety Council News Release.
27. "Consumer Tire Guide."
28. Correspondence from the Mintex Corporation,
Cincinnati, Ohio.
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29. "1981 Tire Guide," Bennett Garfield, Tire Guide, Box
677, Syosset, N.Y.
30. "5 Keys to Better Tire Mileage and Safety," Tire
Industry Safety Council, Washington, D.C., p. 3.
31. Grugett, "Effect of Tire Inflation."
32. NHTSA, Regulatory Analysis.
33. Viergutz, "In-Use Survey."
34. D.M. Coddington, "Inflation Pressure Loss In Tubeless
Tires - Effects of Tire Size, Service and
Construction," Rubber Chemistry and Technology, Vol.
52.
35. Highway Statistics, 1979, Table VM-1, Highway
Statistics Division, Office of Highway Planning, FHWA.
36. Paul S. Francher, James E. Bernard and Lloyd H. Emery,
"The Effects of Tire-In-Use Factors on Passenger Car
Performance, "Society of Automotive Engineers 741107,
1974.
37. P.S. Francher et al, "Steering Controllability
Characteristics," DOT HS-6-01409, Highway Safety
Research Institute, The University of Michigan, Ann
Arbor, Michigan, 1977, Chapter III.
38. Collier, "Effect of Inflation."
39. R. Bruce Michael, "Update on the Fuel Economy Benefits
of Inspection and Maintenance Programs," EPA-AA-
IMS/81-10, U.S. EPA, Ann Arbor, April 1981.
56
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APPENDIX A
CALCULATION OF FUEL ECONOMY AND
TREADWEAR BENEFITS
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APPENDIX A
CALCULATION OF FUEL ECONOMY AND
TREADWEAR BENEFITS
This appendix presents the methodology and assumptions
employed to calculate the fuel economy and treadwear
benefits summarized in Chapter I. Three elements are
needed to calculate fuel economy and treadwear benefits:
The distribution of tire pressure of in-use
vehicles compared to recommended pressure
The distribution of tire pressures after the
inflation program
The change in pressure with time
Each is described below followed by a detailed calculation
of the effect of inflation pressure on fuel economy and
treadwear.
TIRE PRESSURES OF IN-USE VEHICLES
Three sources were found which provided distributions
of in-use tire pressures. The three sources are presented
in Table A-l. The EPA data was taken from the 1977
emission factors program and is from six cities:
Chicago .
Houston
Phoenix
St. Louis
Denver
Washington D.C.
Thus the EPA study data represents a broad cross section
of the different climates and driving conditions in the
U.S.31
Tests conducted by EPA were also selected on the basis
of make and model year to give a representative sample of
U.S. vehicles-in-use. Tests were all made at a uniform
cold temperature of 76°F and corrected to reflect the
mean outside temperature for the month and location of the
test.
A-l
-------
Based on the measurements taken, mean front pressure
was found to be underinflated by 1 psi and mean rear
pressure was low by 2.6 psi. Of the underinflated tires,
mean front underinflation was 4.1 psi, mean rear under-
inflation was 4.9 psi, and mean overall underinflation was
4.5 psi.
The NHTSA data is a compilation of tire pressure
distribution data provided by Uniroyal, Inc., and three
AAA clubs around the country. The figures were gathered
between 1973 and 1976. ^2 The mean pressure according to
this survey was low by 0.2 pounds.
TABLE A-l. Percent of Tires Below and Above
Manufacturers Recommended Pressure
Amount Below
Manufacturers
Recommended Pressure
(psi)
1
2
3
4
5
6
7
8
9
10 -
11
12
13
14
15
16
17
18
TOTAL (Percent)
Average Low
Amount Above
Manufacturers
Recommended Pressure
(psi)
0
1
2
3
Percent of Tires Measured
EPA
Front
8.8
10.0
9.6
7.5
5.8
4.9
3.1
2.3
1.6
0.87
0.86
0.41
0.15
0.25
0.15
0
0.05
0.05
56.6
4.13
Rear
8.8
9.2
8.7
9.5
8.3
6.3
4.7
3.5
3.4
2.1
1.0
1.4
0.66
0.36
0.20
0.15
0.05
0.10
6 8 . 4-
4.87
NHTSA
8.2
9.1
7.3
5.3
3.2
3.4
2.1
2.1
0.9
1.6
0.5
0.45
0.25
0.12
0.12
0.05
0.02
0.05
44.76
Viergutz
Winter
1
\
~]
\
~*
12
7
14
Summer
16
12
8
Percent of Tires Measured
EPA
Front
9.4
8.0
5.9
5.4
Rear
8.5
6.6
4.5
3.1
NHTSA
Viergutz
Winter Summer
A-2
-------
The Viergutz data, part of which was discussed in
Chapter 1, was derived from measurements in parking lots
in Chicago during both the summer and winter. The
vehicles were immobile for at least three hours before the
test. Mean winter pressure was low by about 2.5 psi and
mean summer pressure was low by about 0.8 psi.
The Viergutz and EPA data are in reasonably good
agreement. The NHTSA data shows the same general trend
but indicates less underinflation. Analyses presented in
the remainder of this chapter are based on use of the EPA
data. The EPA data was selected for use because it is
closest to being representative of all vehicles in the
United States throughout the year.
DISTRIBUTION OF TIRE PRESSURES AFTER AN INFLATION PROGRAM
As mentioned at the beginning of this chapter, three
elements are needed to calculate fuel economy and
treadwear benefits, one of which is the distribution of
tire pressures after the inflation program. In
calculating the fuel economy and treadwear benefits
presented in this report, three inflation strategies were
analyzed as follows:
Strategy 1; Inflate all tires to the vehicle .
manufacturer recommended pressures, adjusted for
temperature. The inspector determines the
manufacturer's recommended pressure by reading
the information on the placard usually found on
the door pillar. If the tires are hot the values
are increased 3 to 4 psi.
Strategy 2; Inflate all tires to 3 psi above the
vehicle manufacturer's recommended pressures^
adjusted for temperature. The inspector
determines the vehicle manufacturer's recommended
pressures and adds 3 psi. He makes sure the
resulting pressures do not exceed the maximum
stamped on the tire sidewall. If the tires are
hot the pressure values are increased another 3
to 4 psi.
Strategy 3; Inflate all tires to the maximum
sidewall pressure, adjusted for temperature. The
inspector determines whether the vehicle is on a
list of models that should not be inflated in the
program (because their vehicle recommended tire
pressures are too different from the maximum
sidewall pressures). If the vehicle is not on
the list, the tires are all inflated to the
maximum pressure listed on the tire sidewall. If
the tire is hot, 3 to 4 psi is added to the
pressure.
A-3
-------
In the following sections, the general decline in tire
pressure after vehicles are inspected and the fuel economy
and treadwear benefits are discussed separately for each
of these strategies.
CHANGE'IN PRESSURE WITH TIME
Researchers at Exxon34 Corporation have concluded
that significant inflation pressure loss occurs in sound,
properly mounted tubeless tires, under static or dynamic
conditions, primarily by air permeating through the tire
structure. The principal part of the tire that effects
inflation loss of tubeless tires is the innerliner.
The rate of air loss for a typical in-use tire is
difficult to determine since the loss varies by tire
type. However, industry engineers have indicated that a
decline of 1/2 psi per month is reasonable. Detailed
survey data are not available.
Perhaps more critical to estimating the rate of
pressure loss for tires is estimating the general decline
in tire pressure after vehicles are inspected. For
purposes of analysis, two "pressure loss over time"
scenarios were considered:
Scenario 1; A decline in mean pressure of 1/2
psi per month with no changes in motorist tire
maintenance habitsjUnder this scenario,
motorists would not change their present tire
maintenance habits. Thus the consumer
information program would have no benefit. The
result would be that tire pressures would be
allowed to decrease to the levels they were prior
to the inflation program.
Scenario 2; A decline in mean pressure of 1/2
psi per month followed by reinflation every two
months by the motorist. This scenario represents
a situation where motorists would be influenced
by the consumer information part of the inflation
programmed and would therefore change their tire
maintenance habits (i.e., motorists would inflate
their tires more frequently then they would
normally do). As a result, the pressure loss
over time would be significantly reduced compared
to Scenario 1.
A-4
-------
Given these two scenarios, the following sections
describe the effects on mean fleet tire pressure of each
inflation strategy. The results are summarized in Table
A-2.
TABLE A-2
Increased Yearly Mean Fleet Pressure Due to Different
Tire Inflation Strategies.
Inflate to Inflate to 3 psi Intiate to
Manufacturers Above Manufac- Maximum
Strategy Recommendations* turers recommen- Sidewall
mendations* Pressure*
Effect considering
no benefit from
consumer information 1.1 2.6 4.3
Effect considering a 2.5 4.7 6.8
benefit of consumer
information
*Adjusted for temperature.
Strategy 1; Inflate All Tires to The Vehicle
Manufacturers' Recommendations, Adjusted for Temperature.
According to the EPA data (Table A-l) the average
underinflation of vehicles with underinflated tires is 4.5
psi. After inflation all these tires will be at
manufacturer's recommended pressure and, after decreasing
at 1/2 psi per month, the mean pressure would return to
4.5 psi under specification after nine months, assuming no
benefit from consumer information. Over a year these
tires would show an average inflation 1.69 psi higher than
would have-occurred without the inflation program.* Since
the underinflated tires are 63 percent of all tires, the
average increase in pressure for the year for all tires
would be 1.1 psi.
If an effective consumer information program
encouraged motorists to reinflate their tires every two
months, then with a 1/2 psi per month pressure loss rate,
It is assumed that the tires would be 4.5 psi higher
than without the inflation program at the beginning of
month one and decline linearly to 0 psi higher than
without the program at the end of the month nine. In
months ten, eleven and twelve the tires are assumed to
be 0 psi higher than without the program.
A-5
-------
the average pressure for the year for tires inflated in
the program would be 1/2 psi below vehicle manufacturer's
recommended pressure. This would be 4 psi above the level
that would have occured without the inflation program.
The average increase in pressure for all tires for the
year would then be 2.5 psi. Thus the consumer information
yields an additional 1.4 psi average pressure increase
over the situation with no consumer information benefit.
Strategy 2; Inflate All Tires to 3 psi Above The Vehicle
Manufacturer's Recommendations, Adjusted for Temperature.
With this strategy, instead of the average tire
pressure being increased 4.5 psi for inflated tires as in
strategy 1, the average increase would be 6.1 psi. This
includes inflating all tires less than 3 psi above
manufacturers specifications (about 84 percent of all
tires, from Table A-l). With a decrease of 1/2 psi per
month (the case of no benefit from consumer information),
the average pressure of the inflated tires would return to
the situation prior to inflation after 12 months, and the
tires that were inflated would average 3.1 psi higher than
they would have without the program. Since these tires
are 84 percent of all tires, the average increase in
pressure for the year for all tires would be 2.6 psi.
With an effective consumer information program
resulting in motorists reinflating their tires evey two
months to the pressures used in the inflation program,
then with a 1/2 psi per month pressure loss rate, the
average pressure for the year for tires inflated in the
program would be 5.6 psi above the level that would have
existed without a program. Since these are 84 percent of
all tires, the average increase in pressure for the year
would be 4.7 psi. Thus, in this case consumer information
could yield an additional 2.1 psi average pressure
increase over the situation with no consumer information
benefits.
Strategy 3; Inflate All Tires to Maximum Sidewall
Pressure, Adjusted for Temperature"
To evaluate the effects of this strategy, one needs to
know the distribution of vehicle-in-use tire pressures in
an absolute sense rather than compared to manufacturers
specifications. The EPA survey mentioned earlier12
found that the average in-use tire pressure was 24.8 psi,
1.8 psi below the average pressure specification.
A-6
-------
The EPA analyzed this data further and found that if
all tires were inflated to 32 psi (cold pressure) unless
already above that level, then the fleet average pressure
would be 32.1 psi, 7.3 psi higher than before inflation.
The EPA used 32 psi as the inflation pressure for the
calculation since most of the tires had this as the
maximum pressure indicated on the sidewall. In the
situation with no consumer information impact, a decline
of 1/2 psi per month for the fleet average pressure would
result in an average pressure increase for the year due to
the inflation program of 4.3 psi. (Virtually all the
tires in the program would be inflated).
In the situation where effective consumer information
influenced motorists to inflate their tires every two
months, the overall increase in pressure for the year
would be 6.8 psi (assuming a 1/2 psi decrease per month
and consumers reinflating to the pressure used in the
program). In this case the consumer information would
yield an extra 2.5 psi average pressure increase over the
situation with no consumer information benefits.
THE EFFECT OF INFLATION PRESSURE ON FUEL ECONOMY AND
TREADWEAR
Table A-3 summarizes the results of different studies
of the effect of inflation pressure on fuel economy and
treadwear. The best estimate for the effect of tire
pressure on fuel economy is the Grugett study result of
.33 percent change in fuel economy per psi change for
radial tires and the best estimate for the effect of tire
pressure on treadware life is the Goodrich study result of
1.7 percent change in treadware life per psi for radial
tires. Results are cited for radial tires because they
are becoming the predominant tire on vehicles-in-use and
will be the most frequently encountered tire in state
inspection programs. In addition, these estimates more
closely represent the composition and driving cycles of
the vehicle-in-use fleet.
Combining these estimates with the average inflation
changes computed in the previous section, one can compute
the range of dollar savings provided by each inflation
strategy. These savings and the calculation methodology
are shown in Table A-4. In addition to the estimate noted
above and in the previous section, the results presented
in Table A-4 are based on the following:
A total yearly fuel cost per vehicle of $996
based on:
An average fuel consumption per car of 664
gallons
A-7
-------
TABLE A-3. Effects of Tire Pressure On
Fuel Economy and Treadwear
Researcher
Effect on Fuel
Economy
Effect on
Treadwear
Grugett et al'
Goodyear
7,8
Taylor9
.33% per psi for
radials urban--
highway cycle,
Chevy Nova
.30% per psi for
radials, constant
45 mph, standard
size car
.75% per psi for
bias, constant
45 mph, standard
size car
.38% per psi for
radials on city-
highway cycle, .5%
per psi at 30 mph
cycle, GM X-car
Goodrich
13
Firestone
11
1.7% per psi for
radials
2.4% per psi for
bias*
1% per psi for
truck tires at
2 psi under-
inflation**
* A decrease in shoulder-crown wear ratio may somewhat
reduce this benefit.
r* The Firestone data is not linear and shows an average
treadwear change of 2.5 percent per psi at 4 psi
underinflation.
A-8
-------
A gasoline price of $1.50 per gallon
A total yearly tire cost per vehicle of $66.70
based on:
An average tire cost of $50
An average tire life of 3 years
As shown in Table A-4, given these assumptions, the total
benefit of an inflation program ranges from $4.82 to
$28.72 depending on the type of inflation strategy and the
amount of benefit realized from the consumer information
program.
A-9
-------
TABLE A-4.
Summary of Fuel Economy and Treadwear Benefits by Inflation Strategy
(Dollars Per Motorist Per Year)
Benefit Category
Fuel Economy Benefit
(1) Average Annual Fleetwide
Increase in Tire Pressures
Resulting from Inflation
Program
(2) Percentage Improvement in
Fuel Economy Per psi
(3) Annual Percentage Improve-
ment in Fuel Economy (1 x 2)
(4) Annual Percentage Reduction
in Fuel Consumption**
(5) Annual Fuel Costs per Car***
(6) Annual Fuel Cost Savings
(4x5)
i
}
JTreadwear Benefit
1 (7) Percentage Improvement in
j Treadwear Life Per psi
j (8) Annual Percentage Improve-
ment in Treadwear (1 x 7)
(9) Annual Percentage Improvement
in Tire Costs**
j (10) Annual Tire Costs per Car****
(11) Annual Tire Cost Savings
(9 x 10)
TOTAL BENEFIT
Inflate to Vehicle
Manufacturer's
Recommendations*
Without a
Consumer
Information
Benefit
1.1 psi
.33%/psi
.36%
.36%
$996
$3.59
1.7%/psi
1.87%
1.84%
$66.70
$1.23
$4.82
With a
Consumer
Information
Benefit
2.5 psi
.33%/psi
.83%
.82%
$996
$8.17
1.7%/psi
4.25%
4.08%
$66.70
$2.72
$10.89
Inflate to 3 psi
Above Manufacturer's
Recommendations*
Without a
Consumer
Information
Benefit
2.6 psi
.33%/psi
.86%
.85%
$996
$8.47
1.7%/psi
4.42%
4.23%
$66.70
$2.82
$11.29
With a
Consumer
Information
Benefit
4. 7 psi
.33%/psi
1.58%
1.53%
$996
$15.24
1.7%/psi
7.99%
7.40%
$66.70
$4.94
$20.18
Inflate to
Sidewall
Without a
Consumer
Information
Benefit
4.3 psi
.33%/psi
1.42%
1.40%
. $996
$13.94
1.7%/psi
7.31
6.81%
$66.70
$4.54
$18.48
Maximum
Pressure*
With a
Consumer
Information
Benefit
6.8 psi
.33%/psi
2.24%
2.19%
$996
$21.81
1.7%/psi
11.56%
10.36%
$66.70
$6.91
$28.72
I
H
O
* Adjusted for temperature.
** If the MPG or miles per tire benefits is X, then the yearly consumption benefit is x/(l + x).
*** Based on an average fuel consumption per car of 664 gallons and a gasoline price of $1.50 per gallon.
**** Based on an average tire cost of $50 and an average tire life of 3 years.
-------
APPENDIX B
METHODOLOGY FOR ESTIMATING CHANGES IN PROGRAM COSTS
-------
APPENDIX B
METHODOLOGY FOR ESTIMATING CHANGES IN PROGRAM COSTS
This appendix presents the general methodologies for
estimating the cost impacts of changes in inspection
programs. The first section discusses estimating
procedures for centralized lanes and the next section
discusses cost estimating in decentralized programs.
METHODOLOGY FOR ESTIMATING THE EFFECTS OF PROGRAM CHANGES
ON COSTS PER VEHICLE IN CENTRALIZED PROGRAMS
This section presents certain procedures for rapidly
estimating the effects of program changes on the cost per
vehicle of a centralized inspection program. The
calculations apply to a dedicated centralized facility
where all program costs are covered by the vehicle test
fee. The first subsection below presents rules-of-thumb
for rapid calculations where many parameters of a typical
program are assumed. The second subsection below presents
more detailed formulas that can be used for more precise
calculations.
Rule-of-Thumb Calculations
There are three primary possible effects of a
modification to an existing lane: capital investment, a
change in labor requirements and a change in throughput.
Figures B-l through B-3 present graphs that demonstrate
the implications of these changes. Figure B-l shows the
change in cost per vehicle of adding an additional
inspector to lanes at different total yearly paid
throughput. Total paid throughput is the total number of
vehicle tests per lane on an annual basis from which fees
are collected or assessed. The method illustrated is
applicable to all systems, regardless of their
efficiency. Figure B-2 shows the change in cost per
vehicle for each additional $1,000 of capital investment
in a lane. Figure B-3 shows the increase in cost as a
percent of base fee for various percentage changes in
throughput. The effect of a change in throughput should
be calculated with a base fee that already reflects any
changes in cost due to labor or capital. Figures B-l and
B-2 reflect the following assumptions: five-year capital
depreciation, 12 percent interest, 10 percent profit on
all annual costs, inspector wage of $11,000, 30 percent
fringe cost, 15 percent general and administrative charge
on labor costs, and two months of training for each new
inspector.
B-l
-------
cc
o
m tu
Q. -1
tn
-------
TOTAL PAID THROUGHPUT PER LANE PER YEAR (1000's)
(VEHICLES/YEAR/LANE)
FIGURE B-2. COST OF ADDITIONAL
INVESTMENT*
Assumes five-year capital depreciation, 12 percent
interest, 10 percent profit on annual charges.
B-3
-------
-10
-15 -20
-25 -30
-35
-40 -45
-50
REDUCTION IN THROUGHPUT
(PERCENT)
FIGURE B-3. COST OF A CHANGE IN THROUGHPUT
B-4
-------
The following example illustrates how these graphs can
be used.
Example. A centralized inspection program is
considering a change that will result in an additional
$10,000 of capital investment in each lane, one new
inspector in each lane and a decrease of throughput of
10 percent. The system currently has 100 lanes and
tests 3 million vehicles per year that pay $10 only
upon initial testing. The $10 currently covers all
program costs. The assumptions of Figures B-l and B-2
are reasonable for this system. What should the new
fee be to keep the program self-supporting?
Answer. Since there are 3 million paid tests in 100
lanes each year, the yearly paid throughput is 30,000
cars per lane. Examining Figure B-l, the cost of
adding an inspector per lane is 69 cents per vehicle.
Examining Figure B-2, the cost of $10,000 of
investment per lane is 10.2 cents per vehicle. These
two changes would increase the fee per vehicle from
$10 to $10.79. Examining Figure B-3, the 10 percent
reduction in throughput increases the fee per vehicle
by 11 percent. Thus the $10.79 must be increased by
11 percent to $11.98, and the new fee should be $11.98.
Other Assumptions
To allow the use of other assumptions in the cost
calculations, the following formulas for labor costs and
capital costs are provided. The effect of a change in
throughput (Figure B-3) is not affected by changes in the
cost assumptions.
1. Cost of Adding $1,000 of Capital per Lane
Change in fee (cents per vehicle) _
of adding $1,000 of capital per lane
1,000 x Capitalization Factor x (1 + % Profit /100)
TOTAL PAID THROUGHPUT PER LANE
The percent profit is the amount of profit a
contractor would charge as a percent of annual costs. In
Figure B-2, 10 percent is used so that (1 + percent
profit/100) would be 1.10. In a state-run program profit
would be zero, so that (1 + percent profit/100) would be
1.0.
B-5
-------
Total paid throughput per lane is the number of
vehicle tests per lane per year from which fees are
collected. This number is the same as was used in Figures
B-l and B-2.
The capitalization factor converts a capital
investment into a series of fixed annual costs that must
be reimbursed. Table B-l shows these factors for
different interest rates, different program lengths and
different depreciation periods. Typically, equipment
would be depreciated in five years, buildings at the
number of years the program is expected to run. Land is
usually not depreciated. The factor used in Figure B-2
was .28, corresponding to 12 percent interest and five
year depreciation.
2. Cost of Adding An Inspector to Each Lane
Change in fee (cents per vehicle) of adding
one person per lane:
= Wage x (1 + % Fringe/100) x (1 + % GSA/100) x (I + % Profit/100) ,
PAID THROUGHPUT PER LANE
Wage x (1 + % Fringe/100) x (1 + % Profit/100) x (yj * Capitalization Factor)
PAID THROUGHPUT PER LANE
The wage is the base wage per year paid to the
inspector. In Figure B-l a wage of $11,000 was used.
The percent fringe is the percent of base wage that
accounts for all fringe costs. In Figure B-l, 30 percent
was used so that (1 + percent fringe/100) became $1.30.
The percent G&A is the percent of operating expenses
used to account for general and administrative expenses.
In Figure B-l, 15 percent was used so that (1 + percent
G&A/100) became $1.15.
The capitalization factor is the same as described
previously and is used to amortize the training costs over
the program life.
3. Other Costs
In a more detailed calculation, one might wish to look
at effects on utilities, property taxes, maintenance,
supplies, etc. However, these costs would generally not
change significantly with a lane modification. Also,
costs of headquarters or other supervisory offices may
increase beyond the 15 percent G&A already allowed for.
B-6
-------
TABLE B-l. ANNUALIZED CAPITAL COST FACTORS*
Interest (%)
Program Life
(Years)
Depreciable Life
(Years)
5
10
15
NO
DPR
6 .8
5 10 5 10
.24 .24 .25 .25
.15 .14 .17 .15
.12 .11 .14 .13
.06 .06 .08 .08
10 12
5 10 5 10
.26 .26 .28 .28
.18 .16 .20 .18
.15 .14 .17 .16
.10 .10 .12 .12
14 16
5 10 5 10
.29 .29 .31 .31
.22 .19 .23 .21
.19 .16 .21 .19
.14 .14 .16 .16
^Calculations assume salvage value based on straight line depreciation. Based
on formula:
DPR - PRL
1 - -
FACTOR =
DPR
(H-INT)
PRL
X
INT (1 + INT)
PRL
[I + INT)PRL - 1
Where:
INT
DPR
PRL
Interest rate
Depreciation period
Program life.
B-7
-------
METHODOLOGY FOR ESTIMATING THE EFFECTS OF PROGRAM CHANGES
ON COSTS PER VEHICLE IN DECENTRALIZED PROGRAMS
To estimate the impacts of adding different tests and
equipment to a decentralized program, several pieces of
information are required:
Shop labor rate
Equipment cost
Method and rate of equipment amortization
Time required to perform the test
Number of inspections performed per shop annually.
Once this information is obtained, calculation of the
impacts is relatively straightforward. Since the above
information is likely to vary from shop to shop, it is
recommended that you initially use "average" figures.
Later you can perform a sensitivity analysis to determine
the likely range of impacts by varying these average
figures.
The following methodology assumes that all costs
including the amortized cost of equipment and inspector
labor time will be passed directly to the consumer in
their entirety. In actual practice private garages do not
recover all the costs of performing the inspection; rather
the fee which is ultimately established is usually lower
than the actual cost of inspection. The private garage
performs the inspection "at a loss" in hopes that it will
receive the potential repair business resulting from the
inspection.
While there are no hard and fast rules for how much of
a "loss" the private garage is willing to accept, our
analysis of several existing programs indicates that this
figure ranges between 20 and 40 percent of the true cost
of inspection. Thus by multiplying the total per vehicle
cost impact which will result by employing the methodology
described below by a number between 20 and 40 percent, one
can reasonably estimate the incremental fee a private
gargage might be willing to accept to perform the
additional tests.
The methodology consists of three steps as follows:
Step 1: Given a specific test/equipment
combination, determine the per vehicle labor cost
by multiplying the per vehicle test time by the
shop labor rate per the same unit of time.
B-8
-------
Step 2; Determine the per vehicle amortized
equipment cost by first determining the
annualized equipment cost and then dividing this
cost by the annual number of garage inspections.
Determine the annualized equipment cost using the
following formula:
Annualized Equipment Initial Equipment i(l+i)n
Cost Cost (l+i)u-l
where i equals the interest rate and n equals the
depreciation period (typically five years). The
above formula assumes the salvage value of the
equipment is zero at the end of the depreciation
period.
Step 3; Determine the total per vehicle cost
impact by summing the resuts of Steps 1 and 2.
The impact on vehicle processing time is simply
the per vehicle test tiroe. There is no personnel
impact since it is assumed that all inspections
are performed by one inspector.
Using the above methodology, the impact of any
test/equipment combination can be determined. By simply
varying the input data (i.e., average figures you
employed), the sensitivity of the impacts can also be
determined. It should be noted, however, that the above
methodology does not consider any additional state
administrative costs that might be associated with the
addition of a test/equipment combination. If such costs
are anticipated, the per vehicle cost impact of these
costs can be determined by dividing these costs by the
total number of vehicle subject to inspection.
B-9
------- |