SDSB 79-27
Technical Report
Tire Related Effects on Vehicle Fuel Economy
by
John Yurko
July 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 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, Noise and Radiation
U.S. Environmental Protection Agency
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Abstract
This paper analyzes the fuel economy results from steady-state and
EPA Urban cycle tests that were conducted on the track at the Transport-
ation Research Center of Ohio, during Fall 1978, and steady-state tests
that were conducted on a twin roll dynamometer at the EPA, MVEL during
Spring 1979. Results were obtained for four different sets of radial
tires and four different sets of bias belted tires, and at different
tire pressures.
The results showed that radial tires gave better fuel economy than
bias belted tires on the track. However, bias belted tires gave better
fuel economy on the dynamometer when the vehicle was tested at the same
dynamometer power absorption.
Larger diameter tires gave better vehicle fuel economies than did
smaller diameter tires on both the track and the dynamometer.
The tires tested varied in their effect on fuel economy by as much
as 6%. The two extremes, high cost and low cost tires, produced better
vehicle fuel economy results than the average quality tires.
The vehicle fuel economy rankings all tires were the same for both
steady state results and for results from the LA4 transient cycle.
All radial tires ranked the same between dynamometer and track
tests. Similarly, bias belted tires ranked the same between track and
dynamometer tests. However, in the aggregate case, bias belted tires
improved in their dynamometer ranking with respect to the radials, as
compared to their ranking on the track.
Increasing tire pressure improved fuel economy.
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Foreword
During the Fall 1978, a vehicle was tested on a track at the Trans-
portation Research Center of Ohio (TRC), over steady-state and transient
cycle tests, with several different sets of tires. Subsequently, the
same vehicle and tires were tested at the EPA/MVEL, on a twin roll
dynamometer at steady-state conditions. Although, the major concern of
the test program was to determine the optimum speed sensor location on
the dynamometer (1), enough fuel economy information was obtained to
make various road and dynamometer tire comparisons.
Among the information obtained from this test program were fuel
economy comparisons by; tire type, tire size, tire quality (cost), tire
manufacturer and tire pressure. In addition, the vehicle fuel economy
ranking order of the tires at steady-state conditions is compared to
their ranking order over transient cycles on the track, and these are
compared to the ranking order at steady-state on the dynamometer. This
report presents the results of these fuel economy comparisons.
Summary
On the track tests, the radial tires showed about a 2 to 4% improv-
ement in vehicle fuel economy over the bias belted tires. However, on
the dynamometer, bias belted tires had the better vehicle fuel economy
results, about 0.5% higher than that obtained with the radial tires.
The larger 15" diameter tires showed about a 1 to 2% vehicle fuel
economy improvement over the smaller 14" diameter tires. It should be
noted that the rolling resistance effects and the N/V (engine rpm to
vehicle velocity ratio) effects were not distinguished, and that the net
fuel economy result is reported.
Considering higher cost to indicate higher tire quality, the least
expensive 15" radial tire (Multimile Supreme) showed a 2% vehicle fuel
economy improvement over the moderately priced 15" radial tire (Firestone
721) on the track over the LA4 cycle, but only 0.3% over the most expensive
15" radial tire (Michelin-X). On the dynamometer, the least expensive
radial also showed vehicle fuel economy improvements, over the most
expensive radial, and the moderately priced radial tire, of about 2% and
3% respectively.
In contrast, for the bias belted tires, the most expensive 15" tire
(Goodyear Custom Power) showed the best vehicle fuel economy results
with improvements over, the least expensive 15" bias belted tire (Multimile
Multiglass II), of about 2% during the LA4 cycles and 0.4% during steady-
state tests on the track. As in the case of the radial tires, the
moderately priced bias belted tire (Uniroyal Fastrak) showed the lowest
vehicle fuel economy results, with about 3% lower vehicle fuel economy
than obtained with the most expensive bias belted tires, during LA4
cycles. Only the moderately priced bias belted tires were tested on the
dynamometer. Therefore, no comparison, by cost, among bias tires was
possible for the dynamometer results.
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The fuel economy ranking order, for the radial tires, was the same
for steady-state track tests, LA4 track tests, and steady-state dynamometer
tests. The ranking order of the bias belted tires was also unchanged in
each case. However, the overall ranking order, including both radials
and bias belted tires, was different between the track and dynamometer
tests, due to the improvement in the relative performance of the bias
belted tires on the dynamometer.
Based on LA4 tests, on one set of bias belted tires and one set of
radial tires; higher tire pressures gave higher vehicle fuel economy for
all tests. At 24 PSI, the radial tire showed about a 0.6% fuel economy
improvement and the bias belted tire showed about a 1.4% improvement,
over the 20 PSI tests. At 45 PSI, the radial tire showed about a 4%
fuel economy improvement and the bias belted tire showed about a 10%
improvement, over the 20 PSI tests.
Overall, the tests showed, with statistical significance, that the
radial tires gave better vehicle fuel economy than the bias belted
tires. Increasing tire size or tire pressures improved vehicle fuel
economy. The least and most expensive tires gave better vehicle fuel
economy than the moderately priced tires. However, specific correlation
between tire cost and vehicle fuel economy cannot be obtained from these
tests. The fuel economy ranking remained the same, within tire types,
for all cases. However, combining radials and bias belted tires, the
bias belted tires ranked higher with respect to the radials on the
dynamometer as compared to their relative ranking on the road.
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Introduction
The rolling resistance of the tires can account for about 20 to 30%
of the resistive forces experienced by a vehicle in motion, under normal
operation. In addition, the tires can affect the efficiency of the
vehicle by changing the N/V ratio (engine rpm to vehicle velocity ratio).
A decrease in N/V, in general, moves the engine operation to a state of
greater efficiency and reduced internal losses. Such a change is usually
also accompanied by a sacrifice in acceleration potential.
It has been known that radial tires, in general, give better vehicle
fuel economy than bias belted tires, mainly because of their lower
rolling resistance (1), and possibly a lower N/V effect. Tires of
larger diameter give better vehicle fuel economy than smaller diameter
tires, due to a decrease in both N/V and rolling resistance (1). Increasing
tire pressure improves vehicle fuel economy, due mainly to a decrease in
rolling resistance and to a lesser extent to N/V (2).
Vehicle fuel economy may differ signficantly between tires produced
by different manufacturers (1). This may be the result of tire quality
(tread thickness, tread material, etc.) or tire design (e.g., tread
pattern) affecting differences in the rolling resistance. Also different
tires within a nominal size class may slightly differ in actual size,
which could cause rolling resistance and N/V differences between tires
of different manufacturers. However, tires of the same nominal size
are not expected to differ enough in actual size to cause a significant
vehicle fuel economy difference.
One way to classify tires of the same type and size, but different
manufacturer, is by the cost of the tire. The basis is that "cheaper"
tires may have less tread thickness and thus lower rolling resistance.
On the other hand, a more expensive tire may have a better quality
rubber which could affect the rolling resistance, or perhaps thicker
tread affecting rolling resistance and N/V. At any rate, it has been
suggested that tire cost may be a useful predicter of fuel economy in
tires, although there is little known in this area, at this time.
Discussion
The data of this report were obtained from track fuel economy tests
of a 1976 Mercury Montego. The vehicle was fully warmed up for all
tests. The tires were not warmed up prior to the LA4 tests, but were
warmed up for 20 minutes at 50 mph prior to the steady-state tests.
Fuel consumption per distance was determined by measuring fuel flow with
a fluidyne flowmeter and distance from a fifth wheel. Ambient conditions
were monitored and recorded during each test and were used to determine
the validity of the tests.
Subsequently, the vehicle was tested on a twin roll dynamometer at
the EPA laboratory in Ann Arbor, with six of the eight tires (excluding
the high and low cost bias belted tires) that were tested on the track.
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Only steady-state tests were conducted on the dynamometer. Fuel con-
sumption was determined in the same manner as the road tests, except
that the rear roll revolutions of the dynamometer, were used to determine
the distance traveled.
The entire test program provided several fuel economy comparisons
with different tire parameters which are outlined below and are dis-
cussed in the following sections:
1. Fuel Economy Comparison between Bias Belted and Radial Tires.
1.1 On a test track over the LA4 cycle.
1.2 On a test track over 50 mph steady-state tests.
1.3 On a dynamometer at steady-state.
2. Fuel Economy Comparison between Different Tire Sizes
2.1 On a test track over the LA4 cycle.
2.2 On a test track over 50 mph steady-state tests.
2.3 On a dynamometer at steady-state
3. Fuel Economy Comparison between Tires of Different Manufacturers
3.1 On a test track over the LA4 cycle.
3.2 On a test track over 50 mph steady-state tests.
3.3 On a dynamometer at steady-state.
4. A Comparison of the Fuel Economy Ranking Order between LA4
Tests and Steady-state Tests on the Track.
5. A Comparison of the Fuel Economy Ranking Order between Dyna-
mometer Tests and Track Tests
6. The Effect of Tire Pressures on Fuel Economy
Although the following sections are titled "Fuel Economy Compari-
sons", all the analyses are described in terms of fuel consumption,
which is the inverse of the fuel economy. Since tests under several
different conditions are being compared, a basis for every condition has
been defined as the ratio of the fuel consumption of any tire "t" to the
fuel consumption of tire no. 1 under nearly the same conditions. This
ratio has been given the symbol, RT/1. A listing of each tire and its
assigned number is given in Appendix A.
1. Fuel Economy Comparison between Bias Belted and Radial Tires
1.1 On a test track over the LA4 cycle.
Referring to Table la, the results of the LA4 tests on the track
showed that the radial tires gave an average of 2.5% lower vehicle fuel
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Table 1
Ranking Order of Tires from Lowest
to Highest Fuel Consumption
la. Over LA4 cycles on the track
Tire
No.
4
1
5
2
3
8
6
7
Type
R
R
BB
R
R
BB
BB
BB
Size
15"
15"
15"
15"
14"
15"
15"
14"
Cost
Low
High
High
Mod
Mod
Low
Mod
Mod
Fuel
Consumed
(cc/km)
269.4
270.3
273.6
274.3
279.0
276.5
282.1
285.8
RT/1
0.997
1.000
1.011
1.014
1.028
1.032
1.042
1.058
Std.
Dev.
0.024
0.000
-
0.014
0.016
0.018
0.016
0.022
No.
of Tests
3
3
1
6
5
3
4
7
Mean
Ambient Temp.
58°
65°
68°
59°
52°
59°
57°
61°
Ib. Over steady-state tests on the track
Tire
No.
4
1
5
8
3
7
Type
R
R
BB
BB
R
BB
Size
15"
15"
15"
15"
14"
14"
Cost
Low
High
High
Low
Mod
Mod
Fuel
Consumed
(cc/km)
165.1
172.0
175.2
175.9
172.0
165.1
RT/1
0.960
1.000
1.019
1.023
1.030
1.044
Std.
Dev.
No.
of Tests
1
1
1
1
1
1
Mean
Ambient Temp.
67°
63°
42°
62°
52°
56°
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consumption than did the bias belted tires. Within a particular tire
size, the difference between the "best" radial tire and the "worst" bias
belted tire was more than 4%. Statistical T-tests showed over a 99%
confidence that these differences between radial and bias belted tires
were significant.
1.2 On a test track at 50 mph steady-state.
Referring to Table Ib, the steady-state track tests showed that the
radial tires gave about 2 to 4% lower vehicle fuel consumption than did
the bias belted tires. Although each tire was only tested once at
steady-state on the track, the results are consistent with the LA4 tests
on these same tires.
1.3 On a dynamometer at steady-state.
Since the test program was designed primarily for a road to dyna-
mometer correlation to determine the optimum speed sensor location (3),
the vehicle fuel economy data obtained on the dynamometer was spread
over three different steady-state speeds; 40, 50, and 55 mph and 3
different dynamometer power settings; 10.4, 11.4, and 12.4 HP. A
maximum of 18 tests were conducted on six of the eight tire sets. The
results are listed in Table 2. In order to combine all the data for a
given tire the RT/1 value was computed at each dynamometer condition and
the average value over all the tests was reported.
In contrast to the track results, on the dynamometer, the bias
belted tires exhibited about a 0.4% lower vehicle fuel consumption than
did radial tires, as shown in Table 2. This difference is even higher,
about 2%, when only comparing the two "worst" case radial tires and the
two "worst case bias belted tires. This 2% lower fuel consumption of the
bias belted tires over the radial tires is probably a more representative
number, since the two bias belted tires, which were "best" on the track
were not tested on the dynamometer. These differences were shown to be
statistically significant with over 99% confidence.
2. Fuel Economy Comparison between Different Size Tires
2.1 On a test track over the LA4 cycle.
Referring again to Table la, the 15" tires showed an average of
about 2% lower vehicle fuel consumption than did the 14" tires, over the
LA4 cycle. Comparing identical tires, which differ only in size, the
15" Firestone 721 radial tire (tire 2) gave 1.4% lower vehicle fuel
consumption than did the 14" Firestone 721 (tire 3), and the 15" Uniroyal
Fastrak bias belted tire (tire 6) gave 1.6% lower vehicle fuel consumption
than did the 14" Uniroyal (tire 7). Statistical confidence in these
differences were greater than 98%.
2.2 On a test track at 50 mph steady-state.
Referring again to table Ib, the 15" diameter tires exhibited about
a 4% lower vehicle fuel consumption than did the 14" diameter tires.
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Table 2
Ranking Order of Tires by Fuel Consumption
at Steady-State on the Dynamometer
(lowest to highest fuel consumption)
Using the RT/1 Values
Tire
No. Cost Type
4 Low R
^ Mod BB
1 High R
2 Mod R
]_ Mod BB
3 Mod R
Size
15"
15"
15"
15"
14"
14"
RT/1
0.979
0.992
1.000
1.007
1.008
1.032
Std.
Dev.
0.023
0.018
-
0.019
0.017
0.012
No.
of Tests
18
17
18
17
17
17
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This is consistent with the results from the LA4 tests. However, 4% may
be slightly high, since the "worst" case 15" tires, which were the
identical models as the 14" tires, were not tested under steady-state
conditions on the track.
2.3 On the dynamometer at steady-state.
Comparing only the identical tire models, on the dynamometer the 15"
tires exhibited 2% lower vehicle fuel consumption than did the 14"
tires, as shown in Table 2. Overall the 15" tires exhibited about 3% lower
vehicle fuel consumption. These results area consistant with tests on
the track and have shown statistical significance with greater than 99%
confidence.
3. Fuel Economy Comparison between Tires of Different Manufacturer
(Tire quality or cost)
3.1 On the track over the LA4 cycle.
Referring to Table 3, the low cost 15" radial tire (Miltimile
Supreme) showed about a 0.3% lower vehicle fuel consumption than did the
high cost 15" radial tire (Michelin-X) and a 1.7% lower vehicle fuel
consumption than the medium cost 15" radial tire (Firestone 721).
However, the difference between the low and high cost radial tires was
not statistically significant. In the case of the bias belted tires
over LA4 cycle on the track, the high cost 15" tire (Goodyear Custom
Power) gave the lowest vehicle fuel consumption; 2% lower than the low
cost 15" bias belted tire (Multimile Multiglas II), and 3% lower than
the medium cost 15" bias belted tire (Uniroyal Fastrak), as shown in
Table 4a. These differences were statistically significant with over
95% confidence.
3.2 On a test track at 50 mph steady-state.
As was the case over the LA4 cycle, at steady-state the low cost
radial tire exhibited lower vehicle fuel consumption than did the high
cost radial tire, this time by about 4%, as shown in table 3b. The
medium cost 15" tires were not tested, in either the bias belted or
radial tires, at this condition.
Also consistent with the LA4 tests, for bias belted tires, but in
contrast to the steady-state radial tire results, the high cost bias
belted tire showed a 0.4% lower vehicle fuel consumption than did the low
cost bias belted tire, as shown in Table 4b.
3.3 On a dynamometer at steady-state.
Referring to Table 3c the dynamometer tests were consistent with
the track tests for radial tires, in that, the low cost radial tire
exhibited a 2% lower vehicle fuel consumption than did the high cost
radial and 3% lower fuel consumption than the medium cost radial tire.
The high and low cost bias belted tires were not tested at steady-state
on the dynamometer.
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Table 3
Fuel Economy Ranking Order of Radial Tires
by Manufacturer (Best to Worst)
3a. At steady-state on the track
Tire
No.
4
1
2
3
Tire
Manufacturer
Multimile Supreme
Michelin-X
Firestone 721
Firestone 721
Cost
Cheap
Exp
Mod
Mod
Size
15"
15"
15"
14"
Consumed
Fuel
(cc/km)
165.1
172.0
not
177.2
Std.
RT/1 Dev.
0.960
1.000
tested
1.030
Samples
1
1
3b. Over LA4 cycles on the track
4 Multimile Supreme
1 Michelin-X
2 Firestone 721
3 Firestone 721
Cheap
Exp
Mod
Mod
15"
15"
15"
14"
269.6
270.5
274.2
278.2
0.997
1.000
1.014
1.028
0.026
-
0.014
0.016
3
3
6
5
3c. At steady-state on the dynamometer
4 Multimile Supreme Cheap
1 Michelin-X Exp
2 Firestone 721 Mod
3 Firestone 721 Mod
15"
15"
15"
14"
0.979
1.000
1.007
1.032
0.023
0.019
0.012
18
18
17
17
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4. Comparison of the Relative Fuel Economy Ranking between Tires
Tested Over the LA4 Cycle and 50 mph Steady-state
Looking back at Tables 3a and 3b, it is seen that the radial tires,
that were tested, ranked equivalently for both steady-states and LA4
tests on the road. The same is true for the bias belted tires tested,
as shown in Tables 4a and 4b. Note, however, that tire 2 and tire 6
were not tested at steady-state, yet their ranking showed statistical
significance over the LA4 tests, with respect to the other tires of the
same type. Tables la and Ib show the combined ranking order of bias
belted and radial tires. Of all the tires tested, only adjacently
ranked tires 8 and 3 reversed positions between the LA4 and steady-state
tests. However, these tires differed in fuel consumption by less than
1% and this small difference is not statistically distinguishable. A
general rule for comparing individual tires on these tests is; if the
difference is greater than 1% it most likely has statistical signifi-
cance.
5. Comparison of the Relative Fuel Economy Ranking between Tires
Tested on the Dynamometer and on the Track
Looking again at Tables 3 and 4, no changes in the ranking order,
between track and dynamometer tests, were observed within tire types,
for both radial and bias belted tires. Note, in Table 4c, that bias
belted tires 5 and 8 were not tested on the dynamometer.
Comparing Tables la and 2, it is evident that the bias belted tires
moved up in the ranking from the track tests to the dynamometer tests.
In fact, as was mentioned in section 1, the average results showed that
bias belted tires had lower vehicle fuel consumption on the dynamometer
than did the radial tires.
Comparing individual tires, the bias belted tire 6, which had
higher fuel consumption than radial tires 1, 2, and 8 on the track by
about 1% or greater, had lower consumption than these same tires on the
dynamometer by at least 1%. By the same token, bias belted tire 7 had
about a 3% higher vehicle fuel consumption than did radial tire 3 on the
track, yet a 2% lower consumption than tire 3 on the dynamometer. These
differences were statistically significant with over 99% confidence.
6. The Effect of Tire Pressures on Fuel Economy
6.1 Over LA4 cycles on the track-
Only two tires, 2 and 6, were tested at pressures other than 24 PSI.
These tires were also tested at 20 psi and 45 psi over LA4 cycles on the
track.
The tires were allowed a minimum of 4 hours to cool before capped
air inflation pressures were set. The mean results of these tests are
given in Table 5. As tire pressure increased vehicle fuel consumption
decreased in all cases. For the radial tire number 2, the mean vehicle
fuel consumption at 20 psi was about 0.6% higher than at 24 psi and the
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Table 4
Fuel Economy Ranking Order of Bias Belted Tires
by Manufacturer (Best to Worst)
4a. At steady-state on the track
Tire
No.
5
8
6
7
Tire Manufacturer Cost Size
Goodyear Custom Power Exp 15"
Multimile Multiglas II Cheap 15"
Uniroyal Fastrak Mod 15"
Uniroyal Fastrak Mod 14"
4b. Over LA4 cycles on the track
Fuel
Consumed
(cc/km)
RT/1
Std.
Dev.
175.2 1.019
175.9 1.023
not tested
180.5 1.049
Samples
1
1
1
1
5
8
6
7
Goodyear Custom Power
Multimile Multiglas II
Uniroyal Fastrak
Uniroyal Fastrak
Exp
Cheap
Mod
Mod
15"
15"
15"
14"
273.6
279.1
281.9
286.1
1.011
1.032
1.042
1.058
-
0.018
0.016
0.022
1
3
4
7
4c. At steady-state on the dynamometer
5
8
6
7
not tested
not tested
Uniroyal Fastrack
Uniroyal Fastrak
Mod
Mod
15"
14"
0.992
1.008
0.018
0.017
17
17
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Table 5
Mean Fuel Consumption at Different Tire Pressures for
Tire 2 and Tire 6 over LA4 Cycles on the Track
Tire 2
Tire 6
Pressure
(psi)
45
24
20
45
24
20
Mean Fuel
Consumed
(cc/km)
Std.
Dev.
264.4
274.2
275.8
253.0
281.9
285.9
1.8
3.8
0.0
4.4
4.0
-
No.
of Tests
3
6
2
4
3
1
Mean
Ambient Temp.
55°
59°
66°
57°
57°
70°
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mean vehicle fuel consumption at 45 psi was about 3.6% lower than at 24
psi. For the bias belted tire number 6, the mean vehicle fuel consumption
at 20 psi was 1.4% higher than at 24 psi, and 11% lower at 45 psi compared
to 24 psi. There was over 99% confidence in the differences between 45
psi and 24 psi, and less than 70% confidence in the differences between
24 psi to 20 psi. These effects may be somewhat amplified by ambient
temperature effects, since the mean ambient temperature during the 45
psi test was about 68°, while the mean temperature was about 58°F
during the 24 psi tests, and about 56°F during the 20 psi tests. In
order to determine how much of% an effect the ambient temperature may
have had on the fuel consumption, the vehicle fuel economy for the
individual track tests of each tire, at 24 psi only, has been plotted
versus ambient temperature, as shown in Figure 1. Although the plot is
scattered, there is an apparent decrease in fuel consumption, with
increasing ambient temperature, between 45°F and 75°F. The linear
regression of the data showed about a 0.1% decrease in fuel consumption
for a 1°F rise in ambient temperature. However, one low outlying data
point may be causing an apparent decrease in the actual effect. At any
rate, it does appear that the decrease in fuel consumption that was
observed was due to a combined effect of increased pressure and increased
temperature.
Conclusions
Radial tires produce lower vehicle fuel consumption than did bias
belted tires on the road. However, the same bias-belted tires may
produce lower vehicle fuel consumption than the radials on a twin-
roll dynamometer.
Larger diameter tires produce lower vehicle fuel consumption than
the smaller diameter tires. However, these results were obtained by
metered fuel flow and either fifth wheel or dynamometer roll speed. In
practice, putting larger tires on a vehicle would cause the odometer to
show a lower velocity than the true velocity. If the driver consequently
drove faster this might negate any fuel economy benefits of the larger
tire.
Within tire types, radial tires rank the same with respect to
vehicle fuel consumption over steady-state or LA4 tests on the road, as
well as on the dynamometer. This is also true within bias belted tires
but not in the aggregate case. In the aggregate case, bias belted tires
moved up in their ranking with respect to radial tires on the dynamometer,
as compared to their ranking on the road. It should be pointed out,
however, that this effect is somewhat compensated for by the certification
road load equation and is taken in account by the alternate road load
procedure.
Increasing tire pressure decreases vehicle fuel consumption.
However, the magnitude of the effect from this test program could not be
distinguished between the effect caused by an increase in ambient temperature,
since an increase in ambient temperature also causes a decrease in fuel
consumption.
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Figure 1
Fuel Consumption vs. Ambient Temperature
Fuel
Consumed
(cc/km)
Tire 1
Tire 2
Tire 3
Tire 4
Tire 6
Tire 7
-D
-O
Tire 8 -Q
300 1
O
290 .
280
D
O
A
&
Linear Regression Line
270 •
A
• e
260
45
50
55 60
Temperature (°F)
65
70
75
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Apparently tire quality (indicated by tire cost) can have an effect
on fuel consumption, with tires at the low and high ends of the cost
spectrum showing the lowest vehicle fuel consumption. However, cost
alone is probably not the only factor involved when comparing tires of
different manufacturers.
Recommendation
The results presented in this report are somewhat confounded by
uncontrolled effects of temperature and by the effects of changes in the
vehicle N/V ratio induced by changes in tire size. In order to improve
the confidence in the statements of causality of the observed tire
parameter fuel consumption effects it is recommended that future programs
be carefully designed to avoid, or to identify and correct for the
effects of changes in ambient conditions and the effects of changes in
the vehicle N/V ratio.
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References
1. Glenn D. Thompson, Myriam Torres, "Variations in Tire Rolling Re-
sistance", EPA Technical Support Report for Regulatory Action, LDTP
77-5, October 1977.
2. Richard N. Burgeson, "Tire-Dynamometer Roll Effects", EPA Technical
Report, LDTP 77-4, March 1978.
3. John Yurko, "A Track to Twin Roll Dynamometer Comparison of Several
Different Methods of Vehicle Velocity Simulation", EPA Technical
Report, June 1979.
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Appendix A
Tires
1 = Michelin-X GR78xl5 (most expensive)
2 = Firestone 721 GR78xl5 (moderately expensive)
3 = Firestone 721 GR78xl4 (moderately expensive)
4 = Multimile Supreme GR78xl5 (least expensive)
5 = Goodyear Custom Power Cushion G78xl5 (most expensive)
6 = Uniroyal Fastrak G78xl5 (moderately expensive)
7 = Uniroyal Fastrak G78xl4 (moderately expensive)
8 = Multimile Multiglas II G78xl5 (least expensive)
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