EPA-AA-SDSB-80-04
Technical Report
The Effect of Tire Inflation Pressure
on Vehicle Fuel Economy
by
Bruce Grugett
April 1980
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 deve-
lopments which may form the basis for a final EPA decision, posi-
tion 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
-------
-2-
Purpose
This study investigates the extent of underinflation of in-use
automobile tires and predicts the potential fuel conservation which
would result if this underinflation were corrected.
The report further recommends a simple approach, applicable to
a rapid emissions or safety inspection lane which would reduce the
percentage of vehicles with underinflated tires and would signi-
ficantly reduce U.S. annual gasoline consumption.
Introduction
The technical literature indicates that most vehicles on the
road are operated with underinflated tires. Since tire energy
dissipation increases at lower tire inflation pressure and vehicle
fuel consumption increases with increasing tire energy dissipation,
a vehicle with underinflated tires will consume more fuel than it
would with properly inflated tires. Consequently, a program to
increase average tire inflation pressure on in-use vehicles would
result in significant fuel savings.
The 1977 emission factors program conducted by the EPA MVEL
provided an opportunity to gather tire inflation data from in-use
vehicles. The remainder of this report discusses the analysis of
these data and proposes a simple approach to increase tire infla-
tion pressure and reduce U.S. fuel consumption.
Discussion
The vehicles tested in the 1977 emission factors program were
chosen from vehicle registration lists for six different cities.
These six cities; Chicago, Houston, Phoenix, St. Louis, Denver, and
Washington, D.C. ; represent a broad cross section of the different
climates and driving conditions encountered in the United States.
Test vehicles were selected on the basis of make and model year to
give a sample that was representative of the types of vehicles
actually on the road at the time of the test. Thus the data base
can be considered to represent a microcosm of the United States
with regard to both vehicle population and operating conditions.
All tire pressure data were obtained after the vehicle equil-
ibrated at the test temperature of approximately 76° F. since the
vehicles tested in this program were allowed to stand indoors at
the test temperature for at least eleven hours prior to the test as
required by the Federal Test Procedure for measurement of vehicle
emissions. A correction factor was then applied to the measured
pressure to correct it to the pressure which would have been
obtained at the mean outside temperature for the month and location
of the test.
-------
-3-
This approach of measuring all pressures at the same tempera-
ture has some advantage over the more common approach of measuring
pressures on the street, because measurements will not be affected
by daily variations of the ambient temperature. Consequently, data
obtained by this approach may be more representative of the average
inflation characteristics of the tire. The correction formula
used was the simple ideal gas relation given by:
Pcor = Pmea + Pme^+.14'7 (Tcor - Tlab)
Tlab
where:
Pcor = corrected pressure psig
Pmea = measured pressure at lab temperature psig
Pmea + 14.7 = measured absolute pressure
Tlab = absolute temperature of lab °K
Tcor = mean outside temperature of test city during month of test °K
This ideal gas relation is nearly exact for the temperatures and
pressures encountered in this test and is well within the accuracy
of the tire inflation pressure gauge.
The data were reviewed to identify any entries that were
apparently incorrect. This was considered desireable because the
large size of the data base provided opportunity for key punch or
other transcribing errors. All records that included either a
blank or an indicated pressure of zero were removed since these
were assumed to originate from non-entered data fields. Vehicles
with recommended pressures greater than 32 psi were also excluded.
It was assumed that either these data were in error or that they
were obtained from extra load tires that were not representative of
typical automobile tires since passenger car tires in use at that
time did not have recommended pressures greater than 32 psi.
This screening rejected only about two percent of the data,
leaving 1962 of the original 2,042 vehicles in the data base. The
large remaining sample size and the small rejection percentage both
indicate that the data are reliable.
Data Analysis
All corrected tire inflation pressure data described in the
previous section were analyzed to determine their mean values.
This analysis is summarized in table 1.
The data show that the average front tire has a recommended
inflation pressure of 26.0 psi but is only inflated to 24.9 psi so
-------
-4-
Table 1
Variable Number Minimum Maximum
Mean
Std. Dev.
1.
2.
3.
4.
5.
6.
FSPEC
RSPEC
FPRES
RPRES
FDIF
RDIF
1962
1962
1962
1962
1962
1962
20.000
18.000
10.000
11.000
-19.000
-22.000
32.000
32.000
43.000
46.000
18.000
18.000
25.958
27.318
24.854
24.733
1.1040
2.5846
2.3217
2.8588
3.9902
3.9114
4.4263
4.4834
FSPEC - Front tire recommended pressure
RSPEC - Rear tire recommended pressure
FPRES - Front tire measured pressure
RPRES - Rear tire measured pressure
FDIF - Front tire recommended pressure minus measured pressure
RDIF - Rear tire recommended pressure minus measured pressure
-------
-5-
that the average under inflation is 1.1 psi. For rear tires the
average under inflation pressure is 2.6 psi. Overall the average
tire is underinf lated by 1.8 psi. This compares well with an SAE
paper by Viergutz et al. that found an overall average tire
under inflat ion of 1.4 psi on cars tested in a Chicago parking
lot.J^/ Our data shows average underinf 1 at ion to be slightly greater
than reported in Viergutz 's paper because most of his data was
obtained in the summer when underinf lat ion is less of a problem.
These results are similar to those reported from other surveys.^/
Results of recent EPA tests demonstrate that vehicle fuel
consumption decreases about 0.4 percent for each psi increase in
the tire inflation pressure in the normal operating range. 3/
Consequently, in-use vehicles presently consume about 0.7 percent
more fuel than they would if all tires were inflated to manufac-
tures specifications. For the present vehicle fleet, this repre-
sents a possible fuel savings of 36,000 barrels of fuel per day.
This is based on U.S. light-duty vehicle consumption of 5.22
million barrels of fuel per
Of course, not all passenger car tires are under inflated.
Figures 1 and 2 are histograms showing the distribution of tire
inflation pressures around recommended pressures for front and rear
tires. These figures show that many tires are, in fact inflated to
a pressure greater than the recommended pressure. As table 2
clearly shows these tires offset some of the effect of the under-
inflated tires.
Table 2 shows that 57 percent of all front tires and 68
percent of all rear tires are under inflated. If only underinf lated
tires are considered, front tires are underinflated by an average
of 4.1 psi and rear tires are underinflated by an average of 4.9
psi. Overall 63 percent of all tires are underinflated, and the
average amount of underinf lat ion is 4.5 psi.
This suggests a scheme for correcting inflation pressure that
would save even more fuel than inflating all tires to the recom-
mended pressure. If the pressure of all underinflated tires were
increased to the recommended pressure but the pressure of tires
above the recommended inflation pressure were left unchanged, the
average tire pressure would be 27.6 psi, which would be an increase
of 2.8 psi over the present average inflation pressure of 24.8
psi. This strategy would lower fuel consumption by 1.1 percent and
result in a fuel savings of 57,000 barrels per day.
A program to increase tire pressure could easily be incorpor-
ated into a vehicle inspection lane program. Experience with
previous inspection lane programs indicates that it is important to
keep the test time for each vehicle to a minimum. Therefore, a
strategy that is simpler than inflating each tire to its recom-
mended pressure is desired.
The simplest method would be to use a pressure hose that was
maintained at a constant pressure and that had a check valve to
-------
-6-
HISTOGRAM
MIDPOINT
-22,000
-21,000
-20,000
-19,000
-18,000
-17,000
-16,000
-15.000
-14.000
-13.000
-12,000
-11,000
-10,000
-9,0000
-8,0000
-7,0000
-6,0000
-5,0000
-4,0000
-3,0000
-2.0000
-1.0000
0.
1,0000
2,0000
3,0000
4,0000
5.0000
6.0000
7.0000
8,0000
9,0000
10.000
11.000
12,000
13.000
14.000
15.000
16.000
17,000
18.000
COUNT
0
0
0
1
0
0
1
0
2
5
3
4
7
11
16
32
43
72
92
106
116
156
185
173
196
189
148
114
97
60
45
32
17
17
8
3
5
3
0
1
1
FOR 5.FDIF (EACH X= 5)
•f
+
•f
•fX
•f
+
+x
•f
•fX
+x
+x
+x
+XX
•fXXX
+XXXX
•fXXXXXXX
+XXXXXXXXX
+XXXXXXXXXXXXXXX
+XXXXXXXXXXXXXXXXXXX
•fXXXXXXXXXXXXXXXXXXXXXX
+XXXXXXXXXXXXXXXXXXXXXXXX
.
•fXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
•fXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+XXXXXXXXXXXXXXXXXXXXXXX
+XXXXXXXXXXXXXXXXXXXX
+XXXXXXXXXXXX
+XXXXXXXXX
+XXXXXXX
+XXXX
+XXXX
+XX
+x
+x
•fX
+
•fX
+x
Figure 1
Distribution of Front Tire Pressure
around Recommended Pressure
>,f 0-iless .than^ recommended
< 0 greater than recommended
-------
-7-
HISTOGRAM
MIDPOINT
-22*000
-21.000
-20*000
-19*000
-18*000
-17*000
-16*000
-15*000
-14,000
-13*000
-12,000
-11*000
-10,000
-9*0000
-8,0000
-7,0000
-6,0000
-5,0000
-4.0000
-3,0000
-2.0000
-1,0000
0,
1,0000
2,0000
3,0000
4.0000
5.0000
6,0000
7*0000
8,0000
9,0000
10,000
11,000
12,000
13,000
14,000
15,000
16,000
17.000
18*000
COUNT FOR 6.RDIF (EACH X= 5)
1
0
0
•fX
+
+
0 +
0 -f
0
0
•f
•f
+x
1
0 +
0 +
1 +X
0 +
6 +XX
5 +X
8 4-XX
18 +XXXX
33 -fXXXXXXX
42 -fXXXXXXXXX
58 +XXXXXXXXXXXX
61 -fXXXXXXXXXXXXX
89 +XXXXXXXXXXXXXXXXXX
129 +XXXXXXXXXXXXXXXXXXXXXXXXXX
167 +XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
172 -fXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
180 +XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
170 +XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
187 +XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
162 +XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
123 +XXXXXXXXXXXXXXXXXXXXXXXXX
93 +XXXXXXXXXXXXXXXXXXX
69 +XXXXXXXXXXXXXX
66 +XXXXXXXXXXXXXX
42 -fXXXXXXXXX
20 -fXXXX
28 +XXXXXX
13 +XXX
7 -fXX
4 +X
3 +X
1 -fX
2 +X
Figure 2
Distribution of Rear Tire Pressure
around Recommended Pressure
> 0 less than recommended
< 0 greater than recommended
-------
-8-
Table 2
Tires Inflated over Recommended Pressure
Number Minimum Maximum Mean
FPRES
RPRES
FPRES
RPRES
667
452
Number
1110
1343
22.0
22.0
43.0
46.0
28.4
28.6
Underinflated Tires
Minimum Maximum Mean
10.0
11.0
31.0
31.0
22.58
23.25
Std. Dev.
2.85
2.94
Std. Dev.
3.13
3.37
Measured front and rear pressures for tires inflated above and
below recommended inflation pressure.
-------
-9-
insure no air would ever be let out of any tire. When the hose was
connected to the tire, if the tire pressure was greater than the
hose pressure, the valve would close and no air would be let out of
the tire; but if the tire pressure was initially less than the hose
pressure, the valve would open and allow air to flow from the hose
into the tire until the tire pressure equalled the hose pressure.
In this way tires that initially were inflated to a pressure
greater than the hose pressure would not be affected, but tires
that were initially inflated to a pressure lower than the hose
pressure would be inflated up to the hose pressure.
The remaining question is how to choose this hose pressure. A
higher hose pressure will give a higher resulting average tire
inflation pressure, but it will also result in more tires being
inflated to pressures greater than that recommended by the vehicle
manufacturer. Higher average inflation pressure is desirable
because it decreases tire energy dissipation and hence gives
improved fuel economy. Increased inflation pressure also improves
tire treadlife, with little apparent effect on traction. Drastic
increases in inflation pressure should be avoided, however, since
handling may be affected and impact bruise resistance may be
lowered at very high inflation pressures.5J
Selecting the hose pressure clearly involves making a tradeoff
between increasing fuel economy and increasing tire inflation
pressure above the vehicle manufacture's recommended pressure. A
computer program was used to predict how passenger car tire infla-
tion pressure would be affected by choices of target cold inflation
pressure from 22 to 32 psi. These data are shown in table 3.
Figure 3 is a plot of the average amount of tire pressure
above the recommended pressure versus the percentage reduction in
fuel economy. This figure clearly shows that increasing the hose
pressure results in a reduction in fuel consumption but also
results in a greater amount of inflation over the recommended
pressure. The lower end of the curve is nearly flat which means
that a hose pressure of up to 24 psi will result in reduced fuel
consumption with very little increase in inflation over recommended
pressures. Therefore there is no reason to choose a target cold
inflation pressure less than 24 psi.
If a target cold inflation pressure of 24 psi were used the
average inflation pressure would be increased to 26.0 psi while
barely increasing the average amount of inflation over the re-
commended pressure from 0.98 to 1.0 psi. So a cold inflation
pressure of 24 psi would barely increase overinflation but would
still result in 0.52 percent reduction in fuel consumption which is
a potential fuel savings of 27,000 bbl. per day.
A more aggressive program could use a target cold inflation
pressure of 28 psi. This would increase average inflation pressure
to 28.4 psi while increasing the average amount of overinflation to
2.3 psi. This would reduce fuel consumption by 1.48 percent or
77,000 bbl. of fuel per day.
-------
-10-
Table 3
AVG
STD
%UNDER AVG.UNDER
%OVER AVG OVER
FRONT
REAR
COMBINED
FRONT
REAR
COMBINED
FRONT
REAR
COMBINED
FRONT
REAR
COMBINED
FRONT
REAR
COMBINED
FRONT
REAR
COMBINED
FRONT
REAR
COMBINED
FRONT
REAR
COMBINED
FRONT
REAR
COMBINED
FRONT
REAR
COMBINED
RRONT
REAR
COMBINED
25.371
25.273
25,322
25.635
25.541
25.588
25.996
25.910
25.953
26.466
26.333
26.425
27.036
26.962
26.999
27.708
27.641
27.675
28.464
28,411
28.437
29.294
29.258
29.276
30.188
30.157
30.173
31.116
31,097
31.107
32.077
32.061
32.069
3.207
3.100
3.154
2.950
2.838
2.894
2.651
2.532
2.592
2.324
2.203
2.264
1.993
1.869
1.933
1.673
1,551
1.614
1.384
1.263
1.325
1.134
1.014
1.076
0.924
0.809
0.869
0.756
0.646
0.703
0.621
0.514
0.570
56.065
68.349
62.207
55.861
68.298
62,080
40.775
56.830
48.802
39.908
56.116
48,012
24,465
44,139
34.302
23.394
43.680
33.537
6.932
24.108
15.520
6.830
23.649
15.240
6.116
17.278
11.697
6.116
16.667
11.391
0.000
0,000
0.000
3.246
4.086
3.666.
2.799
3.700
3.250
2.982
3.810
3.396
2.384
3.284
2.834
2.615
3.288
2.951
1.904
2.580
2.242
3.382
3.133
3.258
2.657
2,338
2.498
1,967
1.929
1.948
1.000
1.000
1.000
0.000
0.000
0.000
34.098
23,089
28.593
34.709
23,242
28.976
34.964
23.445
29.205
53.670
37.717
45.693
54.740
38.634
46.687
73.293
52.905
63.099
74.618
53.466
64.042
92.915
75.433
84.174
93.017
75.892
84.455
93.833
82.416
88.124
93.833
83.129
88.481
3.655
3.302
3.479
3.612
3.292
3.452
3.625
3.291
3.458
2.745
2.447
2.596
3.162
2.873
3.018
3.014
2.769
2.891
3.690
3.484
3.587
3.800
3,325
3.562
4.691
4.199
4.445
5.576
4.805
5,191
6,535
5,723
6.129
-------
-^rt-TTT-t t--
. , .
5VO
. i t ..
(0 H'U
cx
. . {J ._ . 4, 4.--.
. . . . o. . . . .
•H
•P
1 • M-l i '
— 1 -H 3-^
1 >-l
2!
O
'
•
60
E>
^
X
o?-O
T.J
* ' • Q)
,_(
£(
6
...... 0) . . , ..
I o ; I"0,
B^
• -..-..-.--••>.-. - - - . ... . ,...,.. , ..... 4 . figure-- j t- --.-t-*--* - -f-.— .^— >-..—.-. ,_> + -.- .-»-4-4- . .. . ... , . , , ...
' . ' • . . .
I ' .. • i ('•''• . ' i 1 . • 1
. . . . . ... Over inflation vs Reduction, i'ri Fuel Consumption ...,:!",< . . , ; , . . , ...
_ . ,. ^ , ,.-..4.4.. . ._„.._,_ * . _„ -f or^Indi-cated" 'Target Inflation Treasure's"" '~* "* --*—*- -r »-*--n- -->•-"-• * <- • t - . , ,,.. ^* , .4 .-.-»
, , ' , •
: ' - '
*t • T 1 ' • "" *
• . * i ' ' '
' ' ' ' I • '
| l , I ' 1 . ' • ,
i * • * ' *
' 4- i. ' I '
' • "*~ 1~' I ' • ^
t
4- , I ^|
t ,"+"... T * '
I . . • , * '
{.,..'• ' . ' , • . ' ' ' '
' '
i
i ' ' I i i ' ' I i I ; 1 ' ' i ' * t i i ' • • i
ID
' ' ' i ' '
i ' 1 ' ' ' : ' ' '
' • ' i ' - . ' '
1 . | ' ' ' . > , ' . ' . ' ' ! ' '
' : ' ' ' I ' | ' 1 1 t ' - ' ' : •
' , ' ' ' ','!''., !•!'•• ' '
-i- • i i t t i- i ' t ' ' ' t 1 l ' 1 ! t i i ' i i . ' i 1 r i 1 i i i • i i . t itl iii i ii - • .1-1 ' 1 )
, , . , t , , ,
• , . . 1 , •
f ' 4 ' ' ' ' 1 SI ' ' ' ' ' •
, , ' ' 3lo • t • ,T1 . ^ ,i11 ' ' *
tr,'. 1 't^'1 ' T,. ' T I
. . ' 9 i ' i • i ' i 1 ; ' t . ' ' '
..- ! i . • . ] . . • ^ ,
Q^ , ^-t- t*,~t. +-,-4-+ + r(4.f^-4-, * ^ +, 4
A/ ;.,{,- ... ' i ';•,.' !
.••.•: J • - , i . f f • • ' ' 1 , : f j
nC • ' . ' + *~t , ' . • f *" ' • ' •
. ' p • . '. i • ' t • i 'i' ''I ' ' I : * < ' ' i ' ' • • .!••!',,
+ ^ * _* i ' * *. - +i_+ -, , <. » + * *. |J.*+.4_ !_* + ' * + +. », , + ili' ; ^ * L 1 L . ' -'.4 1 i :. . :
'*-'''•*•'•*•'*•••* ......... .- . * ,..- , 4 . . 4-4 H. .... 1 4 » t -.-....-, r .. » . -t . ..I , ,. t ... 4 i- .--..-.-,..•., !
. | ;!.'•! | ') ' 1 ' 1 : ' ' 1 ' ' ' ' 1 1 1 1 1 1 I.I 1
?. , 4., ^ ^ . . 0«S- - - - ' '«O, 4 - • 4 - - -1.5". t i 4 . -4 - 3.04 'It ' t f *^'5' * • 4 . , . . 5-O* * 4 H- * ' ' • 3^. .4 , ' • .
1 . 1 ' ' 1 ' • ' ' 1 ' 1 ' . , ,
i : ' i i • ' ' \ ' 1 1 ' ' ' ' . i t ! : ' * ' i ' i ' " i • ' • ! ' • ' ' ' 1 ' • ' ' ' ' ' ' ' ' , ' '
1* t11 CT>*'J * . . • * !r ' i ' ' ' rt''''~Tl1"l ' i f • • ' • ' '
, 4. 4 ,. , 4-. .4^.44.+ ,,.. .4-4 4.4 t/. -reduction- in" fuel 'consumption'* * ^1^"|t^ ' * *--;-: t---r| * . < • • - i • • : , • -
-------
-12-
The most aggressive program would be to use a target cold
inflation pressure of 32 psi. This would raise the average in-
flation pressure to 32.1 psi while increasing the average amount of
inflation above the recommended pressure to 5.4 psi. This pressure,
32 psi, is in general the maximum inflation pressure recommended
for current tires by tire manufacturers and should certainly be
considered a safe pressure. At 32 psi no tire overinflation would
occur that would cause direct tire safety problems. It is poss-
ible, however, that a target cold inflation pressure of 32 psi
could cause adverse handling effects on those vehicles with major
differences in recommended front and rear pressures such as older
Volkswagen sedans (Beetles).
Tire inflation pressures of up to 4 psi above the vehicle
manufacturer's recommended pressure should generally be acceptable
resulting in no signigicant change in vehicle handling or degrada-
tion in the vehicle ride. As shown in table 4, if 24 psi were
chosen as the target pressure only 0.03 percent of all tires would
be increased to a pressure more than 4 psi above the vehicle
manufacturer's recommended pressure. If 28 psi were the target
pressure, only about 2 percent of all tires would be inflated to a
pressure more than 4 psi above their recommended pressure. The
number of tires inflated to more than 4 psi above their recommended
pressures does increase dramatically for target pressures above 28
psi. For a cold target inflation pressure of 32 psi, more than 57
percent of all tires would be inflated to a pressure more than 4
psi above the vehicle manufacturer's recommended pressure. This
dramatic increase is due to the fact that many tires have a re-
commended pressure of 24 psi. This can be seen in figures 4 and 5
which are histograms of recommended front and rear tire inflation
pressures. These figures show that the most common recommended
inflation pressures are 24 psi and 26 psi so that even with a hose
pressure of 28 psi most tires will be inflated above their recom-
mended pressure by only 2 or 4 psi. Thus a hose pressure of 28 psi
gives a generally acceptable inflation pressure while yielding a
significant reduction in fuel consumption.
It should be noted that all pressures discussed thus far are
cold tire inflation presures. In an actual vehicle inspection lane
program, vehicles will be checked with the tires warmed up to
varying degrees. As tires warm up their inflation pressures will
increase by about 4 psi after 45 minutes of operation. On the
average a tire passing the inspection lane will be partially
warmed-up, therefore the hose pressure used should be increased by
about 2 psi over the desired cold inflation pressure to account for
this condition. However, some vehicles passing through the lane
will have cold tires so no hose pressure greater than 32 psi should
be used.
It should be noted that this report only considered fuel
savings from increasing the pressure in passenger car tires. If
light-duty trucks were included in this program addtional fuel
savings would result; however, the pressures discussed in this
report are not appropriate for light-duty trucks.
-------
-13-
Table 4
Target Cold
Inflation
Pressure
Psi
24
26
28
30
32
Avg. Inflation
26.0
27.0
28.4
30.2
32.1
% inflated by
more than 4 psi
over recommended
0.03
0.25
2.06
31.91
57.39
% Fuel Savings
0.52
0.92
1.48
2.20
2.96
Fuel Savings
bbl/Day
27,000
48,000
77,000
114,000
154,000
-------
-14-
HISTOGRAM
MIDPOINT
18*000
19,000
20.000
21,000
22.000
23,000
24,000
25.000
26.000
27.000
28.000
29.000
30.000
31,000
32,000
COUNT FOR l.FSPEC (EACH X= 19)
0 +
0 +
4 +X
5 +X
53 +XXX
12 +X
723 +XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
35 +XX
527 +XXXXXXXXXXXXXXXXXXXXXXXXXXXX
32 +XX
428 +XXXXXXXXXXXXXXXXXXXXXXX
2 +X
19 +X
0 +
121 +XXXXXXX
Figure 4
Distribution of Recommended Front Tire Inflation Pressures
HISTOGRAM
MIDPOINT
18.000
19,000
20,000
21.000
22.000
23,000
24,000
25,000
26,000
27.000
28,000
29,000
30,000
31,000
32,000
COUNT FOR 2.RSPEC (EACH X= 13)
1 +X
0 +
4 +X
1 +X
10 +X
9 +X
503 4-XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
23 +XX
390 +XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
12 +X
506 +XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
9 +X
139 -fXXXXXXXXXXX
15 -fXX
339 +XXXXXXXXXXXXXXXXXXXXXXXXXXX
Figure 5
Distribution of Recommended Rear Tire Inflation Pressures
-------
-15-
Conclusion
Analysis of data collected during the emission factors program
from in-use vehicles show that the average passenger car tire is
underinflated by 1.8 psi. Because underinflated tires have higher
energy dissipation, their presence in the passenger car fleet
causes fuel consumption to be higher than necessary. A program to
add air to underinflated tires could result in a fuel savings of
0.5 to 3 percent.
A simple method to increase inflation pressure suitable
for use in a vehicle inspection lane would be to use a pressure
hose maintained at a constant pressure to inflate tires. A check
valve would be used on the pressure hose to insure that air is
never let out of tires. A program using this method would produce
the following results for the indicated target cold inflation
pressures.
Target Cold
Inflation Resulting Avg. % Reduction in Fuel Savings
Pressure Pressure Fuel Consumption bbl/day
24 26.0 0.52 27,000
28 28.4 1.48 77,000
32 32.1 2.96 154,000
Since vehicles arriving in an inspection lane will have tires
that are warmed-up to varying degrees, 2 or 3 psi should be added
to the desired cold inflation pressure to account for this warm-up
effect.
Recommendat ions
A hose pressure of 30 psi is recommended for any inflation
lane programs. This hose pressure would result in an average cold
inflation pressure of about 28 psi. This is considered to be an
acceptable pressure since:
a) No tire would be inflated above the tire manufacturer's
recommended maximum inflation pressure of 32 psi.
b) Only about 2 percent of all tires would be inflated by
more than 4 psi above the vehicle manufacturer's recommended
pressure.
c) Few vehicles would experience significant ride changes,
therefore, few complaints from the public would be expected.
d) In general it is not expected that vehicle handling would
be significantly affected; however, a few vehicles with large
differences in recommended front and rear tire inflation pressures
might experience some changes in handling. Perhaps these few
vehicles should be excluded from the program.
-------
-16-
References
IJ O.J. Viergutz, H.G. Wakley, L. Dowers "Automobile In-Use Tire
Inflation Survey" SAE 780256.
2/ M. Reineman, G. Thompson, "An Investigation of Fuel Economy
~~ Effects of Tire related Parameters," EPA Technical Report
(Draft April 1980).
3/ J.D. Murrell, Passenger Car Fuel Economy: EPA and Road - A
~~ Report to the Congress - Draft, April 1980.
kj D.B. Shonka, et al., Transportation Energy Conservation Data
Book Edition 3. Oak Ridge, Tenn. Feb. 1979.
_5/ B.L. Collies, J.T. Warchol, "The Effect of Inflation Pressure
on Bias, Bias-Belted and Radial Tire Performance," SAE 800087.
------- |