SDSB 79-13
Technical Report
Fuel Economy Effects of Tires
by
Glenn Thompson
February 1979
NOTICE
Technical Reports do not necessarily represent final EPA decisions
or positions. They are intended to present technical analysis of
issues using data which are currently available. The purpose in
the release of such reports is to facilitate the exchange of
technical information and to inform the public of technical devel-
opments which may form the basis for a final EPA decision, position
or regulatory action.
Standards Development and Support Branch
Emission Control Technology Division
Office of Mobile Source Air Pollution Control
Office of Air, Noise and Radiation
U.S. Environmental Protection Agency
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I. Introduction
Tires have a very significant effect on the fuel economy of a
vehicle. The best known example of this is the fuel efficiency of
radial tires, however, other tire technologies and related factors
can also be important. This report discusses the effects of
various types of tires and tire related parameters on the fuel
economy of a vehicle.
II. General Discussion of the Effects of Tires on Vehicle Fuel
Economy
This section presents a general discussion of the fuel economy
effects of tires, the subsequent sections provide more specific
discussion of the fuel economy effects of various tire tech-
nologies.
Tires affect the fuel economy of a vehicle by their effect on
the force, and hence the work, required from the engine to propel
the vehicle. When the vehicle is operated on a level road the
majority of the energy leaving the engine is dissipated in the
vehicle tires or in overcoming the aerodynamic drag of the vehicle.
Figure 1 shows typical force versus speed requirements for a
vehicle.(1)
The tire dissipative forces are approximately constant with
speed within the nominal operating speed range, however, they
rapidly increase near the maximum design speed of the tire. The
aerodynamic drag forces increase as the square of the velocity of
the vehicle and consequently predominate at high speeds. The drive
train losses are approximately linear with speed and are generally
small compared with either the tire losses or the aerodynamic drag.
Figure 1 indicates that the tire energy dissipation is the
major engine power requirement below approximately 40 mph. Because
a significant portion of all U.S. driving occurs at speeds below 40
mph, and most of the urban driving is in this speed range, the
tire has a very significant effect on the fuel economy of a vehi-
cle.
The previous analysis only considered steady-speed operation
of the vehicle, under more typical, cyclical, vehicle-use con-
ditions the inertia effects of the vehicle somewhat diminish the
tire effects. The EPA emissions and fuel economy driving cycles
are representative of typical urban and highway vehicle use.
Consequently, the fuel economy effects of vehicle tires over these
cycles can be used to predict the fuel economy effects of tires in
typical use. The following sections discuss the specific fuel
economy effects of various tire technologies over the EPA driving
cycles.
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III. Radial Versus Bias Tire Constructions
EPA has empirically investigated the fuel economy effects of
radial versus bias and bias-belt.ed tires over the EPA driving
cycles.(2) The results of this investigation demonstrated that, on
the average, radial tires improved vehicle fuel economy by 5
percent. This result is consistent with other reported data
obtained under similar test conditions.(3)
Currently about 70 percent of the new vehicles sold are
equipped with radial tires; however, only about 40 percent of
the tires sold for replacement are radials.(4) These data indicate
that there is some consumer transition from OEM radial tires to
non-radial replacement tires. Reasonable life expectancies for
tires are about 33,000 miles for radials, 25,000 miles for bias-
belted and 18,000 miles for bias-ply tires. Therefore, the vehicle
which is always equipped with radial tires will require approxi-
mately three sets of tires, the OEM set and two replacement sets to
reach a reasonable vehicle life expectancy of 100,000 miles. The
vehicle which is always equipped with non-radial tires will require
the OEM set and approximately four replacement sets to reach its
100,000-mile distance. If, after the OEM radials are worn, the
consumer chooses to replace these tires with non-radials, approxi-
mately 3 sets of replacement tires will be required to complete the
life expectancy of the vehicle. Using the above replacement
assumptions, the anticipated percentage of aftermarket sales which
are radial tires will match the observed percentage of sales if
approximately 15 percent of the OEM radials are replaced with
non-radial aftermarket tires.
Using the previous tire sales data, the tire life expec-
tancies and the estimated 15 percent aftermarket transitions from
radial to non-radial, it can be calculated that approximately 60
percent of the U.S. mileage is being driven on radial tires. If
the remaining 40 percent were also driven on radial tires the
current U.S. fuel consumption would be decreased by 2 percent.
While consistent selection of radial tires would significantly
improve the national average fuel consumption, an individual
consumer selecting a single set of tires may not incur this bene-
fit. The EPA study which reported an average fuel economy im-
provement of 5 percent with radial tires also reported an average
energy dissipation difference of 25 percent between radial and
non-radial tires. However, energy dissipation variations were
observed among tires of each generic class. In fact, the observed
variation between the "worst case" and the "best" radials was as
large as the difference between the mean of the radials and the
mean of the non-radials.. Unfortunately at the present time, there
is no method to accurately predict a low rolling resistance tire.
In addition, tires are not consistently tested or graded so that
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Force Vs. Speed Requirements for a
Typical Vehicle
200
0)
u
i-i
o
Pn
tfl
a.
•H
CO
to
Total
150 "
100 ••
50 "
Aerodynamic Drag
Tire Losses
Drive Train
20 40
Speed {'mph)
60
FIGURE 1
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Table 1
Energy Dissipation - Inflation Pressure Sensitivity
Tire Cold (%/psi)
G78-14 -3.3
GR78-14 -2.8
H78-15 -3.8
Thermal
Equilibrium (%/psi)
-2.4
-2.1
-2.1
Average -3.35 -2.18
Hot/Cold Average -2.8%/psi
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no large public data base exists from which the consumer could
select optimum tires.
IV. Second Generation Radial Tires
The previous measurements of the fuel economy improvement of
radial tires versus bias or bias-belted tires were obtained from
primarily 1975 model year tires. Recently, higher pressure tires
with reduced energy dissipation characteristics have been intro-
duced. These tires are available on some 1979 model year vehicles,
and are anticipated to be widespread by the 1980 or 1981 model
year.
These second generation tires were originally described by
several names. The current accepted name seems to be "P-metric."
This name refers to the prefixed P on the tire size and the metric
tread width designation of the tire. At the present time, no large
data base on the energy dissipation or fuel economy effect of these
tires is available from a single consistent experimental program.
The empirical data which do exist indicate that these tires dis-
sipate about 20 percent less energy than that characteristically
dissipated by radial tires.(5) Since radial tires typically
dissipate 25 percent less energy than bias or bias-belted tires,
and this results in 5 percent less fuel consumption, it is reason-
able to believe that "P-metric" tires can produce a 4 percent
improvement in fuel economy compared with current radial tires.
The role of the "P-metric" tire on fuel consumption is,
however, significantly confused since "P-metric" is really a tire
size labeling system and is not a generic tire type. These tires
are generally capable of operation at higher inflation pressures
and with reduced fuel consumption compared with current standard
radial tires, however, the "P-metric" designation does not in any
manner assure reduced fuel consumption.
V. Pressure Effects
The current vehicle use literature indicates that vehicle
tires are typically underinflated by 3 to 4 psi.(6) This, of
course, increases the fuel consumption of the vehicle and may
account for some of the discrepancies typically observed between
the EPA measured fuel economies and those obtained by the vehicles
in consumer service.
A small, but consistent, data base does exist on the sensi-
tivity of the tire energy dissipation to inflation pressure.(7)
Table 1 presents the data from four tires, two bias-belted and two
radial, under both cold and thermal equilibrium operating condi-
tions. The average tire energy dissipation sensitivity to in-
flation is approximately*-3.4 %/psi for cold tires and -2.2%/psi
for tires at thermal equilibrium. That is, an increase of 1
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psi in the tire inflation pressure will decrease the energy dis-
sipation of a cold tire by 3.4 percent, or will decrease the energy
dissipation of a tire at thermal equilibrium by 2.2 percent.
The sensitivity coefficient relating fuel economy to tire
energy dissipation is the required link to relate the effects of
tire inflation to fuel economy. Several data sets exist which
provide estimates of this parameter. The EPA study which reported
a 5 percent fuel economy improvement between radial and non-radial
tires and a 25 percent difference in the rolling resistance of
these tires indicates that the fuel economy inflation pressure
sensitivity coefficient is approximately -0.2. That is, a 10
percent decrease in tire energy dissipation would yield a 2
percent improvement in the fuel economy of the vehicle.
A second data set indicated a slightly lower, but similar
sensitivity coefficient of 0.15.(8) These differences could easily
result from vehicle or tire differences. For example, in the case
of a heavy vehicle, or a vehicle with good aerodynamic charac-
teristics the tire effects will be greater. Also as tire energy
dissipation decreases, the sensitivity coefficient will decrease
since the tire losses will represent a decreasing percentage of the
total energy requirements of the vehicle.
The net inflation pressure fuel economy sensitivity coef-
ficient is the product of the inflation pressure energy dissipation
sensitivity coefficient and the energy dissipation fuel economy
coefficient. Using the average values of -2.8%/psi for the in-
flation pressure energy dissipation coefficient and the average
value of 0.17 for the energy dissipation fuel economy sensitivity
coefficient, the average tire inflation pressure fuel economy
sensitivity coefficient is approximately 0.5%/psi.
The typical tire underinflation of 3 to 4 psi therefore
results in a fuel economy penalty of about 2 percent, or about 0.5
mpg for an average current production vehicle.
An additional aspect of the pressure sensitivity coefficient
is that it indicates that an 8 psi increase in tire inflation
pressure would reduce the vehicle fuel consumption by about 4
percent. This is the approximate pressure increase associated with
the "P-metric" tire and also the vehicle fuel economy improvement
associated with these tires. It is therefore concluded that the
fuel economy benefit of this class of tires primarily occurs
because of the increased inflation pressure. This conclusion has
also been reached by other observers.(9)
The trend toward increased recommended tire inflation pres-
sures introduces a significant question. Will vehicle owners
maintain the higher inflation pressures or will they inflate to
current typical tire pressures? If the higher inflation pres-
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sures of the "P-metric" tires are not maintained the discrepancies
between the EPA fuel economy measurements and the fuel economies
obtained by vehicles in consumer use will increase. In addition,
if the increased inflation pressures are not maintained, signifi-
cant portions of the current strategies to reduce U.S. passenger
vehicle fuel consumption will not result in actual fuel savings or
at least will result in a significantly lower reduction in fuel
consumption than anticipated.
V. Tire Wear Effects
Studies of the effects of tire wear are difficult because of
the problem of obtaining matched tires and the the time required to
"wear out" one tire while retaining the control tire. Alternate
approaches used are to buff the tread from a tire to simulate tire
wear, or to obtain tires which have been worn in service. The
former approach does not subject the tire to the side wall flexing
which occurs as the tire is typically worn, while the latter
approach cannot provide the experimentally desireable control tire.
In general, the tire energy dissipation decreases as tire wear
increases. This is logical since the hysteretic energy loss of the
tire rubber is primarily responsible for the tire energy dis-
sipation, and the volume of tire rubber decreases as the tire
wears. In addition, loss of the tread has a significant effect on
the stiffness of the tire. While the number of tires for which
wear effects have been reported is small, the literature indicates
that the energy dissipation of a completely worn tire may be 40
percent less than the energy dissipation when the tire is new.
(10,11)
These wear effects should be noted since they would reduce the
fuel economy improvement a consumer might expect when replacing
worn tires of one type with new tires of a type more fuel effici-
ent.
VI. Conclusions
Two types of conclusions can be reached; those from a consumer
standpoint and those from a national fuel conservation standpoint.
A. Consumer Standpoint
From a consumer standpoint the most cost-effective option is
to buy a tire pressure gauge and to use it. For a typical vehicle
owner traveling about 10,000 miles or more per year on tires
underinflated by 3 to 4 psi, the average annual fuel savings
which would result if this underinflation were corrected is 12
gallons per year. In addition, it is probable that improved tire
life would also occur with better maintenance of tire inflation
pressures. Therefore, it is monetarily advantageous for a vehicle
owner to maintain appropriate tire inflation pressures.
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From the standpoint of tire selection, the only fuel efficient
choice a consumer can make when replacing worn tires is to select
some form of a radial tire. At present, there is insufficient data
available in the marketplace to allow the selection of an optimum
tire from a fuel economy standpoint.
B. National Fuel Conservation Standpoint
Radial tires should be promoted in preference to other types
of tire constructions. In general, this has already occurred for
new vehicles since about 70 percent of current OEM tires are
radials. If all OEM and aftermarket tire sales were radial tires,
the total annual U.S. fuel consumption would be reduced by about 2
percent. This would result in a total annual passenger car fuel
savings of about 1.4 billion gallons.
An additional improvement could also be made if vehicle
manufacturers and consumers would select optimum, low energy
dissipation, radial tires. The vehicle manufacturer has an incen-
tive to choose optimum tires for his vehicle because of the EPCA
fuel economy standards and, at least, the large manufacturers will
have the necessary data available to make this selection. However,
these data are not available to the average consumer. Until this
information is available to the consumer, the choice of optimum
replacement tires is not possible. Consequently, the discrepancy
between the fuel efficiency of production versus aftermarket tires
will probably increase. Since about two-thirds of the normally
expected total useful life of the vehicle will be driven on re-
placement tires, this will impair national fuel conservation
efforts.
Maintenance of recommended tire inflation pressures would also
result in large fuel savings. Correcting the average tire under-
inflation of 3 to 4 psi would also reduce fuel consumption by about
2 percent or more than 1 billion gallons.
Vehicle manufacturers are beginning to increase the recom-
mended inflation pressures for their new vehicles. The EPA regu-
lations, specifically the provision for requesting alternate
dynamometer power absorptions, provide the manufacturers with
credit for these increased inflation pressures in the EPA-DOT fuel
economy programs. If these increased recommended inflation pres-
sures are not followed by the consumer, the increased inflation
pressure benefits will not be seen in the national fuel consump-
tion. This will result in overestimation of the fuel conservation
which is occurring and may increase the discrepancy between the EPA
fuel economy measurements and the fuel economy achieved by the
typical in-use vehicles. It is therefore imperative to encourage
that recommended inflation pressures be maintained.
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References
1. D. A. Clemming and P.A. Bowers, "Tire Testing for Rolling
Resistance and Fuel Economy," Tire Science and Technology,
TSTCA Vol. 2, No. 4 November 1974.
2. G. D. Thompson and M. Torres, "Variations in Tire Rolling
Resistance - A Real World Information Need," Proceedings of the
1977 SAE-DOT Conference Tire Rolling Losses and Fuel Economy -
An R and D Planning Workshop.
3. W. K. Klamp, "Power Consumption of Tires Related to How They
Are Used," Proceedings of the 1977 SAE-DOT Conference, Tire
Rolling Losses and Fuel Economy - An R and D Planning Workshop.
4. S. LaFerre (Editor), "Tire Industry Analysis," Modern Tire
Dealer, January 1978.
5. T. M. Fisher (CM), letter to Charles Gray (EPA), July 25, 1978.
6. 0. J. Viergutz, H. G. Wakely, L. Dowers "Automobile In-Use Tire
Inflation Survey," Society of Automotive Engineers, Paper No.
780256.
7. S. K. Clark, R. N. Dodge "A Handbook for the Rolling Resistance
of Pneumatic Tires," Final Report No. DOT-TSE-78-7 prepared for
the Department of Transporation, Transportation Systems Center.
8. T. M. Fisher op. cit.
9. S. K. Clark, op. cit.
10. W. W. Curtiss, "Low Power Loss Tires," Society of Autmotive
Engineers, Paper No. 690108.
11. J. D. Walter and F. S. Conant "Energy Loses in Tires," Tire
Science and Technology, TSTCA Vol. 2, No. 4, November 1974.
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Public Relations Information Abstract
Fuel Economy Effects of Tires
On the average, radial tires improve the fuel economy of a
vehicle by about 5 percent, or 1 mpg for a 20 mpg automobile. At
the present time, this fuel efficiency combined with the longer
average life of radial tires more than offsets the higher initial
cost of the radial tires. The payback of the radial tire will
increase as fuel cost continues to rise. Consequently, the radial
tire will continue to be the less expensive tire to purchase, on a
cents per mile basis, when obtaining a new car or replacing exist-
ing worn tires.
With any type of tire, the tire inflation pressure has a very
significant effect on the fuel consumption of a vehicle. On a
typical 20 mpg vehicle, tire underinflation of 3 to 4 psi will
increase fuel consumption by about 2 percent or 0.5 mpg. There-
fore, from a consumer standpoint, it is cost effective to buy a
tire pressure gauge and to use it. For a vehicle owner traveling
about 10,000 miles or more per year, proper tire inflation would
typically save 12 gallons of fuel per year. Therefore, it is
monetarily advantageous for a vehicle owner to maintain appropriate
tire inflation pressures.
More extensive information on the fuel economy effects of
tires can be obtained from the 1979 EPA Technical Report, "Fuel
Economy Effects of Tires."
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