EPA-AA-LDTP 78-16
Technical Report
Comparison of Hot to Cold Tire
Fuel Economy
December,1978
by
Myriara Torres
Richard N. Burgeson
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 developments 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 and Waste Management
U.S. Environmental Protection Agency
-------
I. Introduction
As part of a comprehensive tire rolling resistance measurement
program, a tire study was conducted to determine the effect of tire
warm-up on fuel economy. The study was conducted on 33 different sets
of tires at ambient temperature (approximately 75°F). Each set of tires
was installed on the rear two wheels of a fully warraed-up vehicle. The
vehicle was then driven over an FTP driving schedule on a single large-
roll dynamometer and the emissions and fuel economy values were determined
for each phase of the schedule (see Appendix B for explanation of the
FTP).
In this report, the fuel economy changes due to tire warm-up are
summarized and analyzed. In addition, the effect of tire type, size and
manufacturer on the fuel economy values are investigated. Finally, the
effect of tire warm-up on tire rolling resistance is discussed.
II. Program Design
Two vehicles were utilized during this study, a 1971 Ford station-
wagon and a 1971 Vega stationwagon. Tires with nominal size of 14 and
15 inches were mounted on the Ford for test and those with a nominal
size of 13 inches were mounted on the Vega. A description of the tires
can be found in Table A—1 of Appendix A. For the purposes of this
report when the tires were at ambient temperature, they will be referred
to as cold tires.
Each test was conducted on a single large-roll (48 inch diameter)
dynamometer with the power absorption torque set to duplicate Clayton
dynamometer power absorption torques at 50 mph. Only the dynamometerfs
intrinsic inertia (approximately 1800 pounds) was used. Since the
inertia weight was low and held constant for both vehicles, and since
fuel economy is highly dependent on inertia weight during vehicle operation
over the FTP, we then expect the fuel economy changes found in this
study to be larger than they would be under typical test conditions.
The FTP driving schedule used as the test procedure \vras convenient
for measuring tire warm-up effect on fuel economy. There are three
phases in the procedure: a cold transient (505 seconds), a stabilized
phase (872 seconds), and a hot transient phase (505 seconds). The cold
and hot transient phases follow the same speed-time driving schedule,
therefore, the tire warm-up effect on fuel economy was determined by
comparing the fuel economy values obtained during these two phases.
Prior to testing, the vehicle was run for 30 minutes at 50 mph to
achieve a fully warmed-up state. This procedure eliminated the effect
of vehicle warm-up on the fuel economy determinations, so that any
change in the fuel economy values between the cold and hot phases of the
FTP driving schedule should be due to tire warm-up only.
-------
Upon completion of the warm-up, the set of driving tires was then
removed and another set of tires at ambient temperature was installed.
An FTP driving schedule was then conducted and the vehicle emissions
were collected using the Constant Volume Sampler (CVS). The fuel
economy values during each phase were then determined from the vehicle's
emissions according to the carbon balance method. Repeated tests were
conducted on most of the tires only after allowing the set of tires to
cool down to room temperature. Therefore, a total of 66 tests were run.
Driver variability was. minimized by having the same driver conduct the
majority of the tests.
III. Analysis
A. Percent Fuel Economy Improvement of Hot Tires
The percent fuel economy improvement of hot tires over cold tires
was calculated as follows:
,Hot Transient F.E. - Cold Transient F.E..
*• Cold Transient F.E. J X IUU
The average percent improvement for all 66 tests xras 5.4%. Analyses of
variance xjere conducted on the percent fuel economy improvement to
determine the effect of tire type, size and manufacturer. No significant
differences were found due to any of these factors.
This improvement in fuel economy due to warm-up indicates a consis-
tent decrease in rolling resistance also due to the warm-up effect. A
tire rolling resistance study conducted at the EPA laboratories concluded
that a 10 percent change in rolling resistance will yield a 2 percent.
change in the vehicle fuel economy on the road, "if Since the change in
fuel economy observed on the dynamometer is only due to the rear tire
warm-up characteristics, the expected fuel economy improvement on the
road would be twice the dynamometer effect (i.e, 10.8%). Based on the
conclusions of the above EPA report, a rough estimate of the change in
tire rolling resistance during an FTP driven on the road is 50%. It
should be noted however that the lack of inertia simulation during the
dynamometer testing may over estimate the fuel economy improvement due
to tire warm-up characteristics. By increasing the inertia simulation
on the dynamometer and therefore the amount of power being transmitted
through the tire, the temperature changes within the tire would occur more
rapidly, reducing the difference in fuel economy from the cold to hot
transient phases of the FTP. A study investigating the effects of tire
warm-up on fuel economy incorporating representative inertia simulation
is currently underway.
A test program similar to the cold to hot tire test program was
conducted using two sets of elliptical tires at an inflation pressure of
35 PSI and two sets of "equivalent" standard radial tires inflated at 24
PSI. The elliptical tire manufacturer claims that these tires at 35 PSI
are the "equivalent" to radial tires at 24 PSI xjith respect to ride and
handling. 3f The test procedure follox^ed and the equipment used were
identical to those described in the previous section. In this study
-------
each set of tires was tested three times and the percent fuel economy
improvement of hot vs. cold tires was calculated for each test. The
means of the three measurements for each set of tires are presented in
Table 1.
Table 1
Average Fuel Economy Values for Standard Radial
and Elliptical Tires
Tire Tire
ID Manufacturer
26 Uniroyal
27 Firestone
51 Goodyear
52 Goodyear
Tire
Type
Radial
Radial
Elliptical
Elliptical
Hot Cold
Transient Transient
F.E. F.E.
(mpg) (mpg)
% F.E.
Improve-
ment
5.1%
4.5
3.0
2.4
It is clear from the table that elliptical tires provide higher
fuel economy values than standard radial tires. Note that their improve-
ment in fuel economy from cold to hot is not as large as the improvement
for radial tires. The x^eighted city fuel economy value for the vehicle
when mounted with elliptical tires was 14.3 mpg and 14.0 mpg when mounted
with standard radial tires, a 2.1% improvement. It is not known if the
control radial tires would have these same characteristics of lower fuel
economy improvement and stabilization with respect to temperature if
they were tested at the inflation pressure of 35 PSI, although a trend
in that direction would be expected.
B. Analysis of Fuel Economy Values
The fuel economy values obtained during the hot and cold transient
phases were analyzed with respect to tire type, size and manufacturer to
investigate if any of these factors had a significant effect on the fuel
economy values. Vehicle effect was eliminated in the study through the
use of only one vehicle for testing 14 and 15 inch tires and one vehicle
for testing 13 inch tires. The analysis that follows does not include
the results of the elliptical tire study.
The tire type analysis revealed that radial tires had the highest
average cold transient fuel economy values within every tire size grouping.
They also had the highest average hot transient fuel economy values
within every tire size grouping except for 13 inch tires. These values
are shown in Table 2. The differences between the tire type means were
found to be statistically significant within the groupings of 14 inch
and combination of 14 and 15 inch tires only. This difference was due
primarily to the high fuel economy values for the vehicle when equipped
-------
with radial tires. The fuel economy values for each tire type were
compared even more specifically by breaking doxvn the groupings by both
tire size and manufacturer. In 71% of these groupings, radial tires had
a fuel economy value larger than the values for bias belted and bias ply
tires. Therefore, the data does reveal an effect on fuel economy due to
tire type.
Table 2
Average Fuel Economy Values by
Tire Type and Tire Size Grouping
Cold Transient Hot Transient
Tire Bias Bias Bias Bias
Size Radial Belted Ply Radial Belted Ply
13 inches* 37.03 36.83 35.74 38.59 38.90 37.88
14 inches 15.31 15.05 16.17 15.95
15 inches 15.88 15.60 15.67 16.75 16.70 16.43
14-15 inches 15.73 15.13 15.67 16.60 16.06 16.43
*13 inch tires were used on the Vega only.
Note in Table 2 that when 15 inch tires were mounted on the test
vehicle, it obtained higher fuel economy values than when 14 inch tires
were mounted. Analyses of variance showed that this difference is
statistically significant for radial tires during both the cold and hot
transient phases. Therefore, there appears to be an effect of increasing
fuel economy with increase in tire size from 14 inches to 15 inches.
Analyses of variance were conducted to compare the manufacturers'
fuel economy means, however, no significant differences nor trends were
found.
The tire type and tire size results are consistent with the EPA
tire rolling resistance study mentioned earlier in the report. In that
study, radial tires were found to have significantly lower rolling
resistance coefficients than bias tires. Another conclusion of that
study was that for each tire type, the means of the rolling resistance
coefficients decrease with an increase in the tire size. Since a reduction
in rolling resistance coefficients corresponds to an increase in fuel
economy, the results of both studies appear to be in agreement. It
should be noted that the fuel economy effect observed in this study may
be due to the change in N/V ratio when changing from 14" to 15" tires.
-------
IV. Conclusions/Recommendations
The conclusions of this study are:
1. The average percent fuel economy improvement of hot tires over
tires at ambient temperature is approximately 5%. It was found that
this improvement does not change with respect to tire type, size, or
manufacturer. This 5% improvement in fuel economy on the dynamometer
corresponds roughly to a 50% decrease in tire rolling resistance on the
road.
2. The average percent fuel economy improvement of hot over cold
elliptical tires at the inflation pressure of 35 PSI is significantly
less than that of standard radial tires at 24 PSI. These percentages
were approximately 2.5% and 4.5% respectively. It is not yet known if
the lower percent fuel economy improvement for elliptical tires is due
to loiter rolling resistance materials or just the higher inflation
pressure.
3. Vehicles with elliptical tires at the inflation pressure of 35 PSI
were found to obtain better fuel economy than vehicles x*ith standard
radial tires at 24 PSI. In this limited experiment the improvement was
found to be approximately 2% when comparing the weighted city fuel
economy figures. It is not yet knoxra whether the standard radials would
achieve the higher fuel economy values of the elliptical tires if they
were also inflated to 35 PSI, however, a trend in that direction would
be expected.
4. Vehicles equipped with radial tires obtain better fuel economy than
vehicles with bias belted and bias ply tires. Those with 15 inch tires
appear to achieve better fuel economy than those vehicles with 14 inch
tires, however this effect may be caused by a change in the N/V ratio
for the particular vehicle used in this experiment.
The above results indicate a consistent percent change in fuel
economy, and therefore in tire rolling resistance during an FTP regard-
less of tire type and size (a lesser effect was observed for elliptical
tires). Since a single large-roll dynamometer was used for this program,
it can be assumed that if an FTP were driven on the road with the vehicle
and tires under the same conditions similar results would be obtained.
However, it is a well known fact that behavior of tires on the Clayton
dynamometer is not the same as on the road or on the single large-roll
dynamometer. Since the twin small-roll dynamometer requires the tire
to absorb nearly twice the power they would on the road, the fuel economy
effect due to tire warm-up characteristics is small. This fact could
partially explain the current "EPA versus consumer" fuel economy
discrepancy. The consumer typically uses a vehicle for short trips
so that the tire never reaches an equilibrium temperature. Therefore,
the consumer rarely benefits from the lower rolling resistance caused
by increased temperatures.
-------
References
1. "Control of Air Pollution from New Motor Vehicles and New Motor
Vehicle Engines," Federal Register, Vol. 42, No. 124, Tuesday, June
28, 1977.
2. Thompson, Glenn D. and Torres, Myriam, "Variations in Tire Rolling
Resistance," EPA Technical Support Report for Regulatory Action,
October 1977.
3. Eagleburger, John, Manager of Technical Coordination, Product
Quality and Safety, Goodyear Tire and Rubber Company, Telephone
conversation, January 1978.
-------
8
Appendix A
Tire Description and
Fuel Economy Data
-------
Table A-l
Tire Descriptions
Identi-
fication
Number
010
020
050
060
070
080
090
100
110
12A
12B
13B
180
200
210
220
230
240
250
260
270
290
300
320
340
350
360
370
380
390
400
410
420
Manufacturer
Goodyear
Goodyear
Goodyear
Goodyear
Goodyear
Goodyear
Goodyear
Goodyear
Goodyear
B. F. Goodrich
B. F. Goodrich
B. F. Goodrich
Firestone
Goodyear
Uniroyal
Goodyear
General
Uniroyal
Goodyear
Uniroyal
Firestone
Firestone
Uniroyal
Goodyear
Firestone
Uniroyal
Goodyear
Firestone
Uniroyal
Firestone
Uniroyal
B. F. Goodrich
B. F. Goodrich
Size Model/Type
BR70X13 Polyglass Radial WT
BR78X13 Polyglass Radial
HR70X14 Polyglass Radial WT
H78X15 Custom Power Cushion Polyglass
HR78X15 Polyglass Radial
HR70X15 Polyglass Radial WT
HR78X15 Custom Polysteel Radial
B78X13 Cushion Belt Polyglass
H78X14 Polyglass Cushion Bias Belted
HR78X15 Silvertown Steel Radial
HR78X15 Silvertown Steel Radial
H78X15 Custom Long Miler
GR78X15 Steel Belted Radial
HR78X15 Custom Tread Steel Belted Radial
GR78X15 Steel Belted Radial PR6
GR78X15 Custom Tread Steel Belted Radial
GR78X15 Dual Steel II Radial
LR78X15 Steel Belted Radial PR6
ER78X14 Custom Tread Steel Belted Radial
FR78X14 Steel Belted Radial
FR78X14 Steel Belted Radial
HR78X15 Steel Belted Radial
ER78X14 Steel Belted Radial
E78X14 Custom Power Cushion Polyglass
E78X14 Sup-R-Belted Deluxe Champion
B78X13 Fastrak Belted
BR78X13 Steel Belted Radial
BR78X13 Steel Belted Radial
BR78X13 Steel Belted Radial
B78X13 Deluxe Champion
HR78X15 Steel Belted Radial
B78X13 Silvertown Bias Ply
GR78X15 Lifesaver 78 Steel Belted Radial
-------
10
Table A-2 — Continued
Tire
ID
250
110
320
300
270
340
010
100
100
350
410
380
360
370.
020
390
360
370
010
380
. 410
100
350
390
020
Tire Mfg.
Goodyear
Goodyear
Goodyear
Uniroyal
Firestone
Firestone
Goodyear
Goodyear
Goodyear
Uniroyal
Goodrich
Uniroyal
Goodyear
Firestone
Goodyear
Firestone
Goodyear
Firestone
Goodyear
Uniroyal
Goodrich
Goodyear
Uniroyal
Firestone
Goodyear
Tire Type
Radial
Bias Belted
Bias Belted
Radial
Radial
Bias Belted
Radial
Bias Belted
Bias Belted
Bias Ply
Bias Ply
Radial
Radial
Radial
Radial
Bias Ply
Radial
Radial
Radial
Radial
Bias Ply
Bias Belted
Bias Belted
Bias Ply
Radial
Tire
Size
14
. 14
14
14
14
14
13
13
13
13
13
13
13
13
13
13
13
13
13
13
13
13
13
13
13
Cold
Trans
(MPG)
15.3
15.4
15.3
15.3
15.4
14.8
41.1
36.8
36.5
37.2
34.2
36.2
36.7
36.1
37.5
35.0
36.4
37.0
35.7
36.6
36.2
37.8
36.2
36.1
37.0
Hot
Trans
(MPG)
16.2
16.3
16.1
16.2
16.3
15.6
43.9
39.1
38.7
39.2
36.8
37.9
38.5
38.0
38.4
37.4
37.8
37.1
37.4
38.1
37.9
39.3
38.5
38.1
38.8
% F.E.
Imp.
5.9
5.8
5.2
5.9
5.8
5.4.
6.8
6.3
6.0
5.4
7.6
4.7l
4.9
5.3
2.4
6.9
3.8j
0.3;
4.8
4.1
4.7'
;
4.0
6.4:
5.5|
4.9i
-------
11
Table A-2
Tire Fuel Economy Data
Tire
ID
12B
210
180
220
200
060
13B
290
230
110
12A
420
400
080
070
240
090
210
13B
200
060
250
300
260
270
320
050
340
230
180
220
12B
12A
240
070
420
400
290
080
050
260
Tire Mfg.
Goodrich
Uniroyal
Firestone
Goodyear
Goodyear
Goodyear
Goodrich
Firestone
General
Goodyear
Goodrich
Goodrich
Uniroyal
Goodyear
Goodyear
Uniroyal
Goodyear
Uniroyal
Goodrich
Goodyear
Goodyear
Goodyear
Uniroyal
Uniroyal
Firestone
Goodyear
Goodyear
Firestone
General
Firestone
Goodyear
Goodrich
Goodrich
Uniroyal
Goodyear
Goodrich
Uniroyal
Firestone
Goodyear
Goodyear
Uniroyal
Tire Type
Radial
Radial
Radial
Radial
Radial
Bias Ply
Bias Ply
Radial
Radial
Bias Belted
Radial
Radial
Radial
Radial
Radial
Radial
Radial
Radial
Bias Ply
Radial
Bias Belted
Radial
Radial
Radial
Radial
Bias Belted
Radial
Bias Belted
Radial
Radial
Radial
Radial
Radial
Radial
Radial
Radial
Radial
Radial
Radial
Radial
Radial
Tire
Size
15
15
15
15
15
15
15
15
15
14
15
15
15
15
15
15
15
15
15
15
15
14
14
14
14
14
14
14
15
15
15
15
15
15
15
15
15
15
15
14
14
Cold
Trans
(MPG)
15.8
15.8
16.2
15.8
16.1
16.0
15.4
15.7
15.8
15.3
15.8
15.9
16.5
16.0
16.0
16.2
16.0
15.5
15.6
16.0
15.6
15.1.
15.2
15.2
15.5
14.9
15.4
14.6
15.7
15.5
16.1
15.7
16.2
16.1
15.9
15.5
15.8
15.7
15.5
15.3
15.4
Hot
Trans
(MPG)
16.7
16.5
16.6
16.7
16.8
16.6
16.5
16.6
16.4
16.2
16.4
16.4
17.0
16.7
16.8
17.0
16.9
16.4
16.2
16.7
16.7
15.7
15.9
16.3
16.3
15.9
16.4
15.6
16.7
16.6
16.9
16.9
17.0
17.3
17.1
16.7
17.1
16.9
16.5
16.3
16.1
% F.E.
Imp.
5'7i
4.4'
2.5
5.7
4.3
3.8|
7.1
5.7!
3.8 .
5.9i
3.3|
3.1J
3.0]
4.4;
5.0J
4.9|
5.6;
5.8J
3.8
4.4*
7.1 ;
4.0J
4.6!
7-2!
5.2!
6.7 j
6.5 j
6.8j
6.4J
7 i '
.i.0\
7.6,
4.91
7.5J
7.5,
7.7:
8.2:
7.6
6.5
6.5
4.5
------- |