United States Air and Radiation EPA420-P-02-005
Environmental Protection August 2002
Agency M6.GHG.001
&EPA Updating Fuel Economy
Estimates in MOBILE6.3
DRAFT
> Printed on Recycled Paper
-------
EPA420-P-02-005
August 2002
Updating Fuel Economy Estimates in MOBILE6.3
DRAFT
M6.GHG.001
Assessment and Standards Division
Office of Transportation and Air Quality
U.S. Environmental Protection Agency
NOTICE
This technical report does not necessarily represent final EPA decisions or positions.
It is intended to present technical analysis of issues using data that 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.
-------
ABSTRACT
In the previous versions of EPA's MOBILE model, estimates of
fleet fuel economy (by model year and by vehicle class) were used
to help predict refueling losses from gasoline-fueled vehicles
and trucks. The use of these fuel economy estimates was expanded
to help predict particulate emissions from diesel-fueled vehicles
in MOBILE6.1.
In this report, EPA proposes to update those fuel economy
estimates. These updated estimates will then be used to predict
C02 emissions. These updated estimates will still vary only by
model year and vehicle class.
****************************************
Please note that EPA is seeking any input from stakeholders
and reviewers that might aid us in modeling any aspect of fuel
consumption.
Comments on this report and its proposed use in MOBILE6.3
should be sent to the attention of Larry Landman. Comments may
be submitted electronically to mobile@epa.gov, or by fax to (734;
214-4939, or by mail to "MOBILE6 Review Comments", US EPA
Assessment and Standards Division, 2000 Traverwood Drive, Ann
Arbor, MI 48105. Electronic submission of comments is
preferred. In your comments, please note clearly the document
that you are commenting on, including the report title and the
code number listed. Please be sure to include your name,
address, affiliation, and any other pertinent information.
This document is being posted on the MOBILE6 website.
The release of this report will be announced via the MOBILE
listserver. Comments will be accepted for sixty (60) days from
the date this report is released. EPA will then review and
consider all comments received and will provide a summary of
those comments, and how we are responding to them.
-------
*** no ACT ***
DRAFT
Updating Fuel Economy Estimates in MOBILES
Report Number M6.GHG.001
Larry C. Landman
U.S. EPA Assessment and Standards Division
1.0 INTRODUCTION
In recent years, there has been an increased interest in
modeling the emissions of green house gasses (GHG) from mobile
sources. The Assessment and Standards Division is undertaking
two projects to address the needs for a "bottom-up" Mobile
Sources GHG model: a new generation model and an "interim" model
which we are calling "MOBILE6.3" because it is be based on
MOBILE6.
The New Generation GHG model will estimate all four major
mobile source greenhouse gasses: carbon dioxide (C02) , air
conditioning refrigerants, nitrous oxide (N20), and methane
(CH4). It will calculate inventories (which is currently
performed as an extra step outside of MOBILE). Its estimates are
likely to take into account additional factors such as vehicle
speed, effects of inspection maintenance programs, extent of air
conditioning usage, ambient temperature, roadway types, roadway
grades, and vehicle weight (including load) to the extent that
those factors are found to be significant.
While work is proceeding on this New Generation GHG model,
EPA is also working on a less sophisticated approach to serve as
an interim model. This interim model, MOBILE6.3, will be based
on MOBILE6. MOBILE6.3 will add the ability to estimate C02
emissions to the MOBILE model in a simple fashion. Vehicle fuel
economy estimates already in the model, based on vehicle class
and model year, will be updated. We will also add an option for
the user to input these rates, e.g. to model alternative
assumptions about future vehicle fuel economy performance. C02
emission rates will then be calculated from these fuel
consumption values by assuming that all the carbon in the fuel
that is not emitted as HC or CO, is emitted as C02-*
* EPA calculates fuel consumption (gallons per mile) as a linear combination
of HC, CO, and C02 (see 40 CFR 600.113) . Therefore, any one of those four
values (i.e., CC>2) can be calculated given the remaining three.
-------
-2- DRAFT
Previous versions of the MOBILE model have contained
estimates of fuel economy (by model year and by vehicle class).
Those estimates were used to predict refueling losses from
gasoline-fueled vehicles and trucks and, beginning in MOBILE6.1,
to help estimate sulfate particulate emissions.
MOBILE fuel economy estimates have not been revised in
recent years (at least for the gasoline-fueled vehicles and
trucks) and are poorly documented.
In this report, EPA proposes new, updated estimates for fuel
economy for each of the 28 vehicle classes and for each model
year. The 28 vehicle classes used in MOBILE6 are listed in
Appendix A. Although fuel economy is known to be affected by a
variety of factors, the estimates of fuel economy in MOBILE6.3
(like those in the earlier versions of MOBILE) will depend only
on vehicle type and model year. Hence, it will not be
appropriate to use MOBILE6.3 for tasks estimating the C02 effects
of changing speed limits or the presence of an I/M program.
Instead, EPA intends for MOBILE6.3 to be used to model large
scale (e.g., national annual) modeling domains for which C02
variations due to variations in factors such as vehicle speed and
ambient temperatures can reasonably be expected to "average out."
2.0 DATA SOURCES
The data on in-use fuel economy values (by model year and
vehicle class) were obtained primarily from the following
sources:
(1) "Light-Duty Automotive Technology and Fuel Economy Trends,
1975 Through 2001" [1] as well as an earlier version of that
report "Light-Duty Automotive Technology and Fuel Economy
Trends Through 1989. "[2]
(2) "Update Heavy-Duty Engine Emission Conversion Factors for
MOBILE6: Analysis of Fuel Economy, Non-Engine Fuel Economy
Improvements, and Fuel Densities, " [3] and
(3) "National Transportation Statistics 2000. "[4] Used for
estimating motorcycle fuel economy.
Complete references for these reports are contained in Section 5
of this report.
2.1 Fuel Economy of Passenger Cars and Light-Duty Trucks
At the end of each model year, EPA calculates a corporate
average fuel economy (CAFE) for each manufacturer. For decades,
EPA has analyzed the data that were used to calculate these
-------
-3- DRAFT
CAFEs. The results of these analyses were published in a series
of annual "trends reports." (The analyses in those reports do
not include various credits that are part of each manufacturer's
official CAFE.) The most recent of those analyses is the report
entitled "Light-Duty Automotive Technology and Fuel Economy
Trends, 1975 Through 2001. " [1]
The fuel economy values in that report are based on
laboratory data, specifically the Federal Test Procedure (FTP)
and the Highway Fuel Economy Test (HFET). The Department of
Transportation (DOT) uses a composite of these fuel economy
values (i.e., the weighted harmonic average of the FTP and HFET)
to determine compliance with the CAFE standards.
However, EPA has found that the actual in-use fuel economy
falls short of that composite value. Therefore, to more closely
approximate real world fuel economy, the EPA has for several
programs (e.g., on new vehicle labels, in the EPA/DOE Fuel
Economy Guide, and in EPA's Green Vehicle Guide) adjusted these
laboratory results. The multiplicative adjustment factor for the
city (FTP) fuel economy is 0.90 and for the highway (HFET) fuel
economy is 0.78 (see 40 CFR 600.209-95) . Weighting these adjusted
FTP and HFET results produces an "adjusted" composite result that
is about 0.85 times the composite result used in the CAFE
program. These adjusted composite results are the values used in
both the most recent "trends report" [1] and in this report.*
In Appendices B and C, we have reproduced passenger car and
light-truck fuel economies (respectively) from the first table of
the most recent "trends report." [1] EPA believes that these
"adjusted" composite fuel economy values are most appropriate
estimates of the real world passenger car and light truck fuel
economies for each model year (from 1975 through 2001) to use for
MOBILE6.3. The basic assumption here is that average fuel
economy does not change as the vehicles age.
2.1.1 Separating Light-Duty Diesel-Fueled from Gasoline-Fueled
It is necessary to separate the gasoline-fueled vehicles
from the corresponding diesel-fueled vehicles for two reasons.
First, the equation to calculate C02 emissions for the gasoline-
fueled vehicles is different from the one used for diesel-fueled
vehicles. Second, the fuel economy values of the diesel-fueled
vehicles are usually higher than the fuel economy values of the
corresponding gasoline-fueled vehicles.
A recent analysis [5] of the results of the Residential Transportation
Energy Consumption Survey (a 1985 survey of car owners conducted by the
Energy Information Administration of the US DOE) indicates that the
shortfall may be larger than those EPA adjustment factors. However, since
we do not have any adjustment factors superior to the current ones, we
will use them in this analysis.
-------
-4- DRAFT
Appendices B and C categorize the light-duty portion of the
fleet as passenger cars and light trucks; however, in MOBILE6,
the passenger cars (LDVs) and light trucks (LDTs) are divided
into gasoline-fueled and diesel-fueled classes, and the light-
duty trucks are further divided into four weight classes
(Appendix A) . Unfortunately, the recent "trends report" [1] does
not provide that additional break down. Separate calculations
for gasoline and diesel were dropped when the penetration of
diesel-fueled vehicles decreased to a point where their effect on
the fleet average fuel economy was not considered significant.
However, the earlier version of the "trends report" [2] does
have separate calculations of the fuel economies of the gasoline-
fueled and the diesel-fueled fleets. In Appendix E, we compiled
data from Tables 6 and 7 of an earlier version of the "trends
report" [2]. This stratification of fuel economies by fuel type
was limited to the 1978-1989 model year range. In Appendix F, we
"adjusted" those values and combined them with the values from
Appendices B and C. The diesel sales fractions used for these
calculations come from MOBILE6 [6], and are reproduced in Appendix
D. After examining the fuel economies in Appendix F, we observe:
(1) As expected, the fuel economies of the gasoline-fueled
vehicles match the corresponding values for the combined
(gasoline plus diesel) vehicles for the newest model years
due to the small percentage of diesels in the fleet for
these model years.
(2) While the fuel economies of the gasoline-fueled vehicles
(both cars and trucks) are a relatively smooth function of
model year, the fuel economies of the diesel-fueled vehicles
have substantial year-to-year fluctuations. This is a
manifestation of their small sales volume. The low sales of
light-duty diesel passenger cars allow one or two diesel
engines to dominate sales for particular model years (often
different engines for different years).
While the fleet fuel economies for LDVs and LDTs in
Appendices B and C (and hence Appendix F) are calculated for each
model year from 1975 through 2001, the separation by fuel type
(Appendix E) is limited to the 12 model years from 1978 through
1989. To expand these estimates to the larger range of model
years, EPA made the following assumptions based on observations
of the values in Appendix E:
(3) The fleet fuel economies (by model year) of the light-duty
gasoline vehicles (LDGVs) was 0.1 or 0.2 miles per gallon
(mpg) lower than the corresponding combined values. The
difference tended to be 0.2 for the model years with the
largest diesel penetrations and 0.1 for the years with the
smallest diesel penetrations.
-------
-5- DRAFT
Thus, EPA proposes to set the 15 missing fuel economies (in
Appendix F) for the LDGVs to be 0.1 mpg less than the
corresponding values for the LDVs (i.e., combined).
(4) The fuel economy of the 1978 and 1979 model year light-duty
diesel vehicles (LDDVs) averaged 24.0 mpg. EPA proposes to
estimate the fuel economies of the 1975-77 model year LDDVs
as also 24.0.
Similarly, the fuel economy of the 1986-89 model year LDDVs
averaged 32.4 mpg. EPA proposes to estimate the fuel
economies of the 1990-2001 model year LDDVs as also 32.4.
(5) Among the 1986-89 model year LDTs, the penetration of
diesels is comparable to the 1990-2001 model years. In the
1986-89 model years, the fleet fuel economies (by model
year) of the light-duty gasoline trucks (LDGTs) averaged
about 0.1 mpg lower than the corresponding combined values.
Thus, EPA proposes to set the 12 missing fuel economies (in
Appendix F) for the LDGTs in model years 1990-2001 to be 0.1
mpg less than the corresponding values for the LDTs.
(6) For the 1986-89 model years, the fuel economies of the
light-duty diesel trucks (LDDTs) averaged 20.6 mpg. EPA
proposes to set the fuel economies for the 1990-2001 model
year LDDTs equal to the same value.
(7) The diesel penetration in the LDTs for the 1976-77 model
years is comparable to the penetration during the 1978-79
model years. During the 1978-79 model years, the fuel
economies of the LDGTs was within 0.1 mpg of the
corresponding values of the LDTs. Hence, EPA proposes to
set the fuel economies of the 1976-77 model year LDGTs to be
0.1 mpg less that the corresponding LDTs.
The diesel penetration for the 1975 LDTs was comparable to
that of the 1981-82 model years. During those two model
years, the fuel economies of the LDGTs was about 0.4 mpg
less than the corresponding values of the LDTs. Hence, EPA
proposes to set the fuel economy of the 1975 model year
LDGTs to be 0.4 mpg less that the corresponding LDTs (i.e.,
11.2 mpg)
(8) The fuel economies of the 1978-79 LDDTs averaged 18.0 mpg.
EPA proposes to set the fuel economies of the 1975-77 LDDTs
equal to that value.
Using the proposals in these preceding points numbered 3
through 8, EPA developed estimates for the 60 values missing from
Appendix F. Inserting those values into Appendix F produces
Appendix G.
-------
-6- DRAFT
2.1.2 Separating Light-Duty Trucks by Weight Classes
None of the "trends reports" distinguishes the LDT fuel
economies by the weight classes in Appendix A. However, we can
calculate those values from Appendix G if we know:
(1) the relative number of trucks in each of the weight classes
and
(2) the fuel economy ratios by model year (i.e., the LDT12 fuel
economy divided by the fuel economy of the corresponding
LDT34).
In MOBILE6*, the LDT12 comprise, on average, about three-
fourths of all of the LDTs [6].
We can estimate those ratios of light-duty fuel economy (by
model year) by making use of a well known fact, namely, that the
test weight of the vehicle is one of the major factors in
predicting its fuel economy (in dynamometer testing). However,
the distinguishing characteristic between the LDT12 and the LDT34
is the gross vehicle weight rating (GVWR), not the test weight
(or even the curb weight).** (The GVWR of the LDT12 is 6,000
pounds and lighter, and the GVWR of the LDT34 is from 6,001 up
through 8,500 pounds.) However, the GVWR does not correlate
perfectly with either the vehicle test weight or the curb weight.
We do not have the average test weights of these two truck
classes on a national basis. However, examining the test weights
of the trucks in the Mobile Source Observational Data Base
(formerly the Emission Factors Data Base) [7], we note that the
mean test weight of those LDT12 trucks was 3,607 pounds, while
the mean test weight of those LDT34 trucks was 4,539 pounds.
Then, in Table 14 of the earlier trends report [2], the
average fuel economy is calculated for each light-duty truck test
weight class for each model year (from 1978 through 1989). While
the fuel economies vary by model year, the ratios of the fuel
economy of the 3,500 pound test weight class divided by the
corresponding fuel economy of the 4,500 pound test weight class
ranges between 1.165 and 1.403, with a mean value of 1.3045. (If
we were to use the data in that table to first estimate, by
interpolation, the fuel economies that would be associated with
* In MOBILES (see References 6, Table 4), for the 1975-96 model years, the
ratio of the number of LDT12s to the number of all LDT ranged between 52.1
percent and 79.5 percent (with an average of 75.13 percent) . For the
purpose of these estimates, we will use a constant ratio of three-fourths.
** The "curb weight" is the actual weight of the vehicle in operational
status with all standard equipment and weight of fuel at nominal tank
capacity. The "gross vehicle weight rating" (GVWR) is a value defined by
the vehicle manufacturer which includes the total weight of the vehicle
plus the weight of the fluids, driver and the maximum recommended payload.
-------
-7- DRAFT
3,607 and 4,539 test weights, and then calculate those fuel
economy ratios. We obtain ratios that range between 1.173 and
1.362, with a mean value of 1.2830. Alternatively, rather than
using interpolation, a third approach to estimating the fuel
economies associated with 3,607 and 4,539 test weights would be
to assume that the ton-miles per gallon is relatively constant in
the range of each of those two test weights. This third approach
yields ratios that range between 1.241 and 1.420, with a mean
value of 1.3403.) Each of those three approaches reaches the
same conclusion, the ratio of the fuel economy of the LDT12 to
the fuel economy of the LDT34 (for each model year) is about 1.3.
Applying that fuel economy ratio of 1.3 for LDTs to the
values in Appendix G allows us to estimate the fleet fuel
economies (by model year) for the LDT12 and LDT34 classes. The
results of those calculations are given in Appendix H.
To check these values (in Appendix H) for reasonableness, we
note that the predicted fuel economies of the passenger cars are
higher than the fuel economies of the LDT12 which are themselves
higher than the fuel economies of the LDT34. Comparing values in
Appendix H to the values in Tables 2 and 3 (Section 2.2), we
observe that the fuel economies of the LDT34 are higher than the
fuel economies of the heavy-duty trucks. Thus, these predictions
appear to be internally consistent.
2.2 Analysis of Heavy-Duty Fuel Economy
The major source of fuel economy estimates for heavy-duty
vehicles is an analysis prepared for EPA by ARCADIS Geraghty &
Miller, Inc. [3] of survey data (summarized in Appendix I) from
the 1992 Truck Inventory and Use Survey (TIUS). (That survey has
since been renamed the "Vehicle Inventory and Use Survey," i.e.,
"VIUS.") In that report, ARCADIS developed equations to predict
the fuel economy for each class of heavy-duty vehicles for a
limited range of model years.
In particular, fuel economy data from 59,046 heavy-duty
vehicles (other than buses) from model years 1983 through 1992
were used to predict heavy-duty fuel economy for four years into
the future (i.e., model years 1993 through 1996). The report
does not discuss how to estimate fuel economies for model years
earlier than 1983 nor for model years after 1996.
Since actual in-use fuel consumption data (rather than
dynamometer test results) were used in this analysis, no
adjustment factors were required (unlike the light-duty analyses
in Section 2.1).
For each class of heavy-duty vehicles, the regression
analyses modeled the logarithm of the average fuel economy (for
-------
-8-
DRAFT
each vehicle class in Appendix I) as a linear function of the
logarithm of the model year. That is:
LN(Fuel Economy) =A + (B * LN (Model Year - 1900) )
where "A" and "B" are given in Table 1 (below).
Table 1
Coefficients for Modeling Heavy-Duty
In-Use Fuel Economy
Class
HDV2b
HDV3
HDV4
HDV5
HDV6
HDV7
HDV8a
HDV8b
Gasoline- Fueled
A
0.1253
0.1157
0.0409
0.4416
0.0338
0.1277
0.0647
B
0.9624
0.9632
1.1902
0.6348
1.2015
0.8909
1.0285
Diesel -Fueled
A
0.1072
0.0989
0.5020
0.2474
0.5336
4.0206
0.15485
0.0119
B
1.0506
1.0450
0.6598
0.8078
0.6117
0.1374
0.8194
1.3742
Using the coefficients from Table 1, we then calculated the
estimated fuel economy for each of 15 heavy-duty vehicle classes
for 14 model years from 1983 through 1996. Those 210 values are
reproduced in the following tables (Tables 2 and 3). These
values duplicate the corresponding values (from Tables 12 and 14
in the ARCADIS report) for model years 1987 through 1996. (The
contractor was not asked to include model years prior to 1987 in
its tables.)
In Table 1, no equation was provided to estimate the fuel
economy of the HDGV8B trucks.* EPA proposes estimating the fuel
economies of that missing class by examining the fuel economies
of the diesel-fueled trucks (in Table 3). In Table 3, we can
calculate the ratio of the fuel economies of the class 8B trucks
to the 8A trucks (i.e., HDD8B / HDD8A) for each model year. EPA
proposes to multiply the estimated fuel economies of the
gasoline-fueled 8A trucks by those ratios.
* Those class HDGVSb trucks are rare or non-existent, and MOBILES estimates
a population of zero for them. However, some fuel economy estimate is
required for completeness.
-------
-9-
DRAFT
Table 2
Projected Gasoline Heavy-Duty Vehicle Fuel Economies (mpg)
Model
Year
83
84
85
86
87
88
89
90
91
92
93
94
95
96
Weight Classes:
2b
8.81
8.91
9.01
9.11
9.22
9.32
9^42
9.52
9^62
9.73
9.83
9.93
10.03
10.13
3
8.16
8.26
8.35
8.45
8.54
8.63
873
8.82
a92
9.01
9.11
9.20
9.30
9.39
4
7.87
7.98
8.09
8.21
8.32
8.43
8.55
8.66
8.78
8.89
9.01
9.12
9.24
9.35
5
7.30
7.35
7.41
7.47
7.52
7.58
7.63
7.68
7.74
7.79
7.85
7.90
7.95
8.01
6
6.83
6.93
7.03
7.13
7.23
7.33
743~
7.53
763~
7.73
7.84
7.94
8.04
8.14
7
6.54
6.62
6.69
6.76
6.83
6.89
ase
7.03
7AO
7.17
7.24
7.31
7.38
7.45
8A
6.09
6.17
6.24
6.32
6.39
6.47
a54
6.62
670
6.77
6.85
6.92
7.00
7.07
8B
5.43
5.55
5.64
5.75
5.91
6.00
aos
6.19
a29
6.38
6.47
6.57
6.67
6.76
Table 3
Projected Diesel Heavy-Duty Vehicle Fuel Economies (mpg)
Model
Year
83
84
85
86
87
88
89
90
91
92
93
94
95
96
Weight Classes:
2b
11.13
11.27
1141
11.55
11.69
11.83
11.97
12.11
1Z26
12.40
1Z54
12.68
12.82
12.96
3
10.01
10.14
1027
10.39
10.52
10.65
10.77
10.90
iTos
11.15
1128
11.41
11.53
11.66
4
9.27
9.34
9.41
9.49
9.56
9.63
9.70
9.77
9.85
9.92
9.99
10.06
10.13
10.20
5
8.78
8.87
8.95
9.04
9.12
9.21
9.29
9.38
9.46
9.54
9.63
9.71
9.80
9.88
6
7.96
8.02
aos
8.14
8.20
8.25
8.31
8.37
a42
8.48
834
8.59
8.65
8.71
7
7.38
7.39
7^40
7.41
7.43
7.44
7.45
7.46
7^47
7.48
7^49
7.51
7.52
7.53
8A
5.79
5.84
53Q
5.96
5.96
6.03
6.10
6.17
024
6.31
ass
6.45
6.52
6.59
8B
5.16
5.25
5^33
5.42
5.51
5.59
5.68
5.77
5^86
5.95
ao3
6.12
6.21
6.30
-------
-10-
DRAFT
Also, there has been litigation concerning the effects of
off-cycle operations of some heavy-duty diesel trucks (classes 3
through 8b) on NOx emissions. The question is whether there was
also an effect on fuel economy. An EPA analysis of the phase-in
of these vehicles suggests the effect (if any) on fuel economy
would be small until 1992; therefore, we expect little if any
effect on the regression equations.
Most (but not all) of the values in Tables 2 and 3 follow
these expected patterns:
Fuel economy tends to decrease as weight increases.
Fuel economy tends to improve with the newer vehicles.
Diesel-fueled vehicles tend to achieve higher fuel economy
than their gasoline-fueled counterparts.
Since the values in Tables 2 and 3 are simply smoothed
regressions of the TIUS (raw) results, it is not surprising that
they would follow these expected patterns. However, the raw
results in Appendix I (i.e., summaries of the actual TIUS results
on which the regressions were based) also follow these expected
trends.
The contractor performed similar analyses on a (different)
data set of fuel economies of various types of bus. Reproducing
Table 15 from that contractor report, yields the following table
of bus fuel economies (for model years 1987 through 1996).
Table 4
Estimated Bus Fuel Economies (mpg)
Model
Year
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
<=== Diesel -Fueled ===>
Transit Intercity School
3.43 4.64 6.29
3.47 4.69 6.28
3.51 4.75 6.27
3.55 4.80 6.25
3^59 485 a24
3.63 4.91 6.23
3.67 4.96 6.22
3.71 5.01 6.20
375 5^07 6/19
3.79 5.12 6.18
<=== Gasoline- Fueled ===>
Transit Intercity School
3.11 3.64 6.18
3.15 3.68 6.21
3.19 3.72 6.24
3.22 3.76 6.27
3^26 3^80 aSO
3.30 3.85 6.33
3.33 3.89 6.37
3.37 3.93 6.40
3^40 3^97 a42
3.44 4.01 6.45
-------
-11-
DRAFT
However, the six classes in Table 4 (above) do not exactly
match the bus classes in MOBILE6. In MOBILE6 (see Appendix A),
there are only three bus classes:
HDGB (all gasoline-fueled buses),
HDDBT (diesel-fueled transit and urban buses]
HDDBS (diesel-fueled school buses).
and
Therefore, we must combine the two transit and urban diesel-
fueled classes as well as combine all three gasoline-fueled
classes.
EPA noted in the MOBILE6 report on fleet distributions [6]
the preponderance of gasoline-fueled buses is school buses. In
that same report, when EPA determined the conversion factors to
be used for HDDBT class, EPA weighted together the diesel-fueled
transit buses equally with the diesel-fueled intercity buses.
Following this logic, EPA proposes combining the fuel
economies for those six bus classes into the three MOBILE6 bus
classes (from Appendix A) by using:
the fuel economies of gasoline-fueled school buses for the
HDGB,
the fuel economies of diesel-fueled school buses for the
HDDBS, and
the harmonic average of the fuel economies of diesel-fueled
transit buses with the diesel-fueled intercity buses for the
HDDBT.
This leads to the following table:
Table 5
Estimated Bus Fuel Economies (mpg)
(by MOBILES classes)
Model
Year
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
HDDBT
3.94
3.99
404
4.08
413
4.17
4.22
4.26
4.31
4.36
HDDBS
6.29
6.28
a~27
6.25
a~24
6.23
6.22
6.20
6.19
6.18
HDGB
6.18
6.21
a~24
6.27
aso
6.33
6.37
6.40
6.42
6.45
-------
-12- DRAFT
2.3 Motorcycles
The third source of fuel economy estimates is an annual
report prepared by the Bureau of Transportation Statistics of the
U.S. Department of Transportation (DOT). In the current edition
of that report [4], the DOT compiled and published a variety of
transportation statistics.
In this DOT analysis (specifically Table 4-22, entitled
"Energy Intensity of Passenger Cars, Other 2-Axle 4-Tire
Vehicles, and Motorcycles"), DOT estimates the total VMT in each
of 13 calendar years (i.e., 1970, 75, 80, 85, 90, 91, 92, 93, 94,
95, 96, 97, and 98) as well as the corresponding amounts of
gasoline consumed. These motorcycle VMT and fuel consumption
values are duplicated in the first three columns of Table 6.
Dividing the VMT (column 2) by the amount of fuel consumed
(column 3) produces an estimate of fleet fuel economy (i.e., the
fourth column of Table 6). These estimated fuel economies of the
in-use motorcycle fleet vary over the very narrow range of 49.56
to 50.26 miles per gallon (with a mean of 50.006). Based on this
tiny variation in annual fleet fuel economies with no obvious
trend (neither increasing nor decreasing with model year) in the
in-use fleet fuel economies, EPA proposes to fix the fuel
economies of this class at a constant 50.0 mpg for each model
year. However, it has been suggested that DOT simply assumed a
fuel economy of 50 mpg and then used that assumption along with
its VMT estimates to predict the total gasoline consumed. If
that were the case, then our attempt to use the DOT numbers to
calculate fuel economy would be circular reasoning, and we would
simply be accepting DOT'S assumption that the fuel economy of on-
road motorcycles averaged 50.0 mpg for each model year.
-------
-13-
DRAFT
Table 6
DOT Estimates of
of the In-Use
Fuel Consumption
Motorcycle Fleet
Calendar
Year
1970
1975
1980
1985
1990
1991
1992
1993
1994
1995
1996
1997
1998
VMT
(millions
of miles)
3,000
5,600
10,200
9,100
9,600
9,200
9,600
9,900
10,200
9,800
9,900
10,100
10,300
Gasoline
Consumed
(millions of
gallons)
60
113
204
182
191
184
191
198
205
196
198
202
205
Calculated
Fleet Fuel
Economy
(mpg)
50.00
49.56
50.00
50.00
50.26
50.00
50.26
50.00
49.76
50.00
50.00
50.00
50.24
However, another source of fuel economy estimates [8] also
assumes that motorcycle fuel economy is constant 50 mpg. This
tends to validate our calculated values.
In a third source of fuel economy estimates of motorcycles,
the DOT estimated [9] that the average fuel economy of on-road
(only) motorcycles was about 43 mpg, the average fuel economy of
off-road (recreational) motorcycles was about 59 mpg, and the
average fuel economy of dual-purpose (i.e., both on-road and off-
road) motorcycles averaged between 85 and 119 mpg. These values
also tend to validate our value of 50 mpg for the average of all
on-road motorcycles in MOBILE6.3
3.0 EXPANDING MODEL YEAR RANGES
In Sections 2.1 through 2.3, we developed estimated fuel
economies for each of the 28 vehicle classes for a range of model
years. MOBILE6.3 requires that those estimates be expanded to
cover every model year from 1951 through 2050.
-------
-14-
DRAFT
To expand those fuel economy estimates to the full range of
model years, EPA proposes:
(1) to use the fuel economy estimated (in Section 2) for the
earliest model year for all preceding model years (for each
vehicle class) and
(2) to use the fuel economy estimated (in Section 2) for the
latest (newest) model year for all subsequent model years
(for each vehicle class).
4.0 COMPARING RESULTS WITH FUEL CONSUMPTION ESTIMATES
There are a number of estimates of highway fuel consumption
that are based on measuring production of gasoline and diesel
fuels. A comparison of these "top down" estimates with these
proposed MOBILE6.3 "bottom up" estimates would serve to validate
the fuel economy estimates in this report.
The preceding estimates of fuel economy are all given in
units of miles per gallon. Thus, if estimates of total vehicle
miles traveled (VMT) are known, then total fuel consumption can
be estimated by dividing VMT by the corresponding fuel economy
estimates.
4.1 Comparison with DOT Fuel Consumption Estimates
In a report recently published by DOT [4, Tables 4-11 through 4-15],
estimates of VMT (in millions of miles) and fuel consumption (in
millions of gallons) are given by vehicle type for the 1998
calendar year. These values are reproduced below in Table 8:
Table 8
DOT Estimates of VMT and Fuel Consumption
By Vehicle Type for 1998
Vehicle Types
Passenger cars
Motorcycles
Other Vehicle s with 2-Axles
and 4-Tires
Single-Unit Trucks with 2-Axles
and 6 or More Tires
Combination Trucks
Bus
Totals:
VMT
(millions
of miles)
1,545,830
10,260
866,228
67,894
128,159
6,996
2,625,367
Fuel
(millions of
gallons)
72,209
205
50,579
9,741
21,100
1,049
154,883
-------
-15-
DRAFT
In this report, we have developed unique estimates for 20
truck classes (four light-duty* and 16 heavy-duty); however, in
Table 8, there are only three truck types. And those three types
are not based on gross vehicle weight ratings (GVWR) as are the
MOBILE truck types. There is not a clear match between those two
groups. In this comparison, we will simply merge these three
truck types into a single "ALL Truck" group. Using the VMT
weighting factors in MOBILE6.3, we can aggregate the MOBILE6.3
fuel economy estimates into those same groups.
Modifying Table 8 by combining the trucks into a single
category and adding the corresponding MOBILE6.3 aggregated fuel
economies produces Table 9. Dividing the VMTs** by the MOBILE6.3
fuel economies yields estimates of fuel consumption.
Table 9
Estimates of Annual VMT and Fuel Consumption
By Vehicle Type for 1998
Vehicle Types
Passenger Cars
Motorcycles
ALL Trucks
Bus
Totals:
DOTEs
VMT
(millions
of miles)
1,545,830
10,260
1,062,281
6,996
2,625,367
timates
Fuel Used
(millions of
gallons)
72,209
205
81,420
1,049
154,883
MORN CC 1 CotimotQo
VMT
Fractions
52.54%
0.65%
46.53%
0.28%
VMT**
(millions
of miles)
1,379,298
17,130
1,221,588
7,351
2,625,367
Fuel
Economies
(mpg)
23.41
50.00
13.16
5.32
Fuel Used
(millions
of gallons)
58,922
343
92,809
1,381
153,455
Thus, using the fuel economies in this report, the estimate
of fuel (gasoline plus diesel) used in on-road (highway) vehicles
in 1998 was 153.455 billion gallons which is within one percent
of the DOT estimate of 154.883 billion gallons. Although those
estimates of total fuel consumption are close, the estimates of
fuel consumption by vehicle type do vary more due primarily to
differences in estimates of VMT. The data in Table 9 are
displayed graphically in Figure 1 (on the following page).
* In Appendix A, there are actually six light-duty truck classes; however,
this report develops unique fuel economies for only four of them. (We
combined the LDGT1 and LDGT2 to form the LDGT12.
the LDGT3 and LDGT4 to form the LDGT34.
Similarly, we combined
** MOBILE does not produce VMT estimates. The VMT fractions generated by
MOBILE were multiplied by the total VMT (2,625,367 million miles) from DOT
to obtain the MOBILE6.3 VMT values in Table 9.
-------
-16-
DRAFT
Figure 1
Comparison of Estimates of Annual Fuel Consumption
By Vehicle Type for 1998
DOE Estimates
MOBILES.3
T otal
ALL Trucks
Cars
Bus
Motorcycle
4.2 Comparison with OAR and DOE CO2 Estimates
As explained in the introduction of this report, MOBILE6.3
uses the fuel economy estimates (along with the estimates of HC
and CO emission factors) to calculate C02 emission factors.
Using these C02 emission factors and the estimated VMTs from
Table 9, we calculated the C02 emissions of the on-road (highway)
fleet. Those estimates of fleet C02 emissions are given in Table
10. For comparison, Table 10 includes "top down" estimates of
C02 emissions from a recent analysis of greenhouse gas (GHG)
emissions by EPA's Office of Atmospheric Programs (which is
within EPA's Office of Air (OAR)) [10].
-------
-17-
DRAFT
Table 10
Estimates of Annual CO2 Emission
By Vehicle Type for 1998
(millions of metric tons)
Vehicle Types
Passenger Cars
Motorcycles
ALL Trucks
Bus
Totals:
From GH
CO2 from
Gasoline-
Fueled Veh.
670.16
1.83
373.74
2.20
1,047.93
G Report
CO2 from
Diesel-
Fueled Veh.
7.69
229.74
9.16
246.59
MOBILE6.3
CO2 from
Gasoline-
Fueled Veh.
491.59
2.50
520.45
2.10
1,016.64
Estimates
CO2 from
Diesel-
Fueled Veh.
1.71
299.51
10.92
312.14
Thus, using the fuel economies in this report (and assuming
a total fleet VMT of 2,625,367 million miles), the MOBILE6.3
estimate of C02 emissions produced by gasoline-fueled on-road
(highway) vehicles in 1998 (1,016.64 million metric tons) was
only 3.0 percent less than this "top down" model predicted.
Similarly, the MOBILE6.3 estimate of C02 emissions produced by
diesel-fueled on-road (highway) vehicles in 1998 (312.14 million
metric tons) was 26.6 percent higher than the "top down" model
predicted. And, the MOBILE6.3 estimate of total C02 emissions
(1,328.78) was only about 2.6 percent higher than the "top down"
model prediction of 1,294.52 million metric tons.
A second source for "top down" estimates of C02 emissions is
from the Energy Information Administration (EIA)* of the US
Department of Energy. EIA's estimates of C02 emissions (for
calendar year 1998) from gasoline-fueled vehicles is 1,124
million metric tons and from diesel-fueled vehicles is 310
million metric tons. Combining these estimates with those from
Table 10 produces Table 11 (on the following page).
* The fuel consumption estimates from EIA are available on their website
(http://www.eia.doe.gov/).
-------
-18-
DRAFT
Table 11
Estimates of Annual CC>2 Emission
By Vehicle Type for 1998
(millions of metric tons)
Source of Estimates
EIA
MOBILE6.3
EPA's GHG Report
CO2 from
Gasoline-
Fueled Veh.
1,124
1,017
1,048
CO2 from
Diesel-
Fueled Veh.
310
312
247
TOTAL
1,434
1,329
1,295
The data in Table 11 are displayed graphically in Figure 2.
Figure 2
Comparison of Estimates of Annual CO2 Emission by Fuel Type for 1998
(millions of metric tons)
1,500
o
**
,u
E 1,000
o
L=
1
.5 500
H
H
(SI
EIA
MOBILES.3
EPA's GHG Report
CO2 from Gasoline-
Fueled Veh.
CO2 from Diesel-
Fueled Veh.
T otal C02
From the data in Table 11 (or from Figure 2), we observe:
The MOBILE6.3 prediction of total C02 emissions (from diesel
plus gasoline) is between the estimates for the two "top
down" models of C02 emissions.
-------
-19- DRAFT
The MOBILE6.3 prediction of C02 emissions from gasoline-
fueled vehicles is less than the estimates for the two "top
down" models of C02 emissions. However, it is within three
percent of the estimate of the estimate from the GHG report,
and still within 10 percent of the EIA estimate.
The MOBILE6.3 prediction of C02 emissions from diesel-fueled
vehicles is more than the estimates for the two "top down"
models of C02 emissions. However, it is within a fraction
of a percent of the estimate of the estimate from the EIA.
Thus, this "bottom up" approach produces estimates (of fuel
consumption and C02 emissions) within a few percent of those
produced by three "top down" approaches (based on total national
consumption of fuel).
These MOBILE6.3 "bottom up" predictions are very sensitive
to estimates of the total miles driven which in turn are
calculated by using estimates of population size (by model year
and by vehicle type) and estimates of the average miles driven
annually (again, by model year and by vehicle type). Shifting
those estimates of either fleet size or VMT could easily account
for the differences among the estimates.
Also, the "top down" approach (based on total national
consumption of fuel) has its own set of uncertainties. For
example, even if the exact amount of gasoline consumed nationally
were known, determining the portion of that amount to allocate to
on-road (versus non-road) uses is based on several assumptions
(each containing some degree of uncertainty).
Nevertheless, the two approaches appear to confirm that the
estimates of fuel economy are accurate given the quality of data
available.
-------
-20- DRAFT
5.0 REFERENCES
1. K. Hellman and R.M. Heavenrich, "Light-Duty Automotive
Technology and Fuel Economy Trends, 1975 Through 2001," EPA
Report Number EPA420-R-01-008, September 2001. (Available
at: http://www.epa.gov/otaq/fetrends.htm)
2. R.M. Heavenrich and J.D. Murrell, "Light-Duty Automotive
Technology and Fuel Economy Trends Through 1989," EPA Report
Number EPA/AA/CTAB/89-04, May 1989.
3. L. Browning, "Update Heavy-Duty Engine Emission Conversion
Factors for MOBILE6: Analysis of Fuel Economy, Non-Engine
Fuel Economy Improvements, and Fuel Densities," (prepared by
ARCADIS Geraghty & Miller, Inc. for the USEPA) EPA Report
Number EPA420-P-98-014 (MOBILE6 Report Number M6.HDE.002),
May 1998. (Available at:
http://www.epa.gov/otaq/models/mobile6/m6hde002.pdf)
4. "National Transportation Statistics 2000," Published by the
Bureau of Transportation Statistics of the U.S. Department
of Transportation, Report Number BTS01-01, April 2001.
(Available at: http://www.bts.gov/btsprod/nts/)
5. M. Mintz, A.D. Vyas, L.A. Conley, "Differences Between EPA-
Test and In-Use Fuel Economy: Are the Correction Factors
Correct?" Paper No. 931104, Transportation Research Board,
Washington, DC., 1993.
6. T. Jackson, "Fleet Characterization Data for MOBILE6:
Development and Use of Age Distributions, Average Annual
Mileage Accumulation Rates, and Projected Vehicle Counts for
Use in MOBILE6," EPA Report Number EPA420-R-01-047 (MOBILE6
Report Number M6.FLT.007), September 2001. (Available at:
http://www.epa.gov/otaq/models/mobile6/r01047.pdf)
7. EPA's "Mobile Source Observational Data Base" (MSOD).
(Information at: http://www.epa.gov/otaq/models.htmftmsod)
8. "California Motor Vehicle Stock, Travel And Fuel Forecast,"
California Department of Transportation, Office of Traffic
Improvement, November 1993.
(Available at: http://ntl.bts.gov/DOCS/cal.html)
9. S.C. Davis, L.F. Truett, and P.S. Hu, "Fuel Use for Off-Road
Recreation: A Reassessment of the Fuel Use Model," Office
of Highway Information Management, U.S. Department of
Transportation, DOT Report Number ORNL/TM-1999/100, July
1999, pages 20-22.
10. "Inventory of U.S. Greenhouse Gas Emissions and Sinks:
1990-1998," Office of Atmospheric Programs, U.S. EPA, EPA
Report Number EPA 236-R-00-001, April 2000, Table 2-8, page
2-10.
-------
-21-
DRAFT
Appendix A
Vehicle Classes in MOBILES
No.
1
2
6
_
8
10
11
12
""Ts"
""14""
""Ts"
""Te"
1Z
""Ts"
""l"9"
20
"21
22
23
24
25"
26
"27"
28
Abbrev.
LDGV
TDGTI
TDGT2
LDGTS
TDGT4
HDGV2b
HDGV3
HDGV4
HDGV5
HDGV6
HDGV7
_____
HDGVSb
LDDV
LDDT12 .......
HDDV2b
HDDV3
HDDy4
HDoys
HDoye
HDDy7
HDOySa
HDDSb
HDGB
HDDBT
HDDBS
LDDf34
Description
Light-Duty Gasoline Vehicles (Passenger Cars)
_____ _____ _ _______
LVW)
_______
LVW)
___
LVW)
___
Ibs. LVW)
_________
______
_______
lbs^ GyWR)
Heavy-Duty Gasone yehicles (8501-1
Class 3 Heavy-Duty Gasoline Vehicles (10,001-14,000 Ibs.
GVWR)
Class 4 Heavy-Duty Gasoline Vehicles ^^QQ^^^QQQ |bs
GVWR)
Class 5 Heavy-Duty Gasoline Vehicles (16,001-19,500 Ibs.
GVWR)
Class 6 Heavy-Duty Gasoline Vehicles (19,501-26,000 Ibs.
GVWR)
Class 7 Heavy-Duty Gasoline Vehicles ^Q^OI -33,666 Ibs.
GVWR)
Class 8a Heavy-Duty Gasoline Vehicles (33 ^.55 ^555 |bs
GVWR)
^^ ^ j_|eavy:puty "Gasoline "^hicjes70v
Light-Duty Diesel Vehicles (Passenger Cars)
Light-Duty Diesej Trucks land 2 (6-6,666 jbs! GVWR)
Class 2b Heavy-Duty ^DJesej Vehicles (850J -10, 666 Ibs! GVWR)
Class 3 Heavy-Duty ^Diesel ^hicjes~(iap0^14^00lbs7GW§
Class 4 Heavy-Duty Diesel Vehicles 0
Class 5 Heavy-Duty Diesel Vehicles (16,661-1^56^
Class 6 Heavy-Duty ^DJesej ~
Class 7 Heavy-Duty ^DJesej
Class 8a Heavy-Duty Diesel Vehicles (33,001-60^
Class 8b Heavy-Duty Diesel Vehicles (Over 60,000 Ibs. GVWR)
Motorcycles (Gasoline)
Gasoline Buses (School, Transit and Urban)
Diesel Transit and Urban Buses
Diesel School Buses
________ _
-------
-22-
DRAFT
Appendix B
Passenger Car Fuel Economy by Model Year
(Reproduced from Table 1 of Reference 1)
Model
Year
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
Lab
Composite*
15.8
17.5
18.3
19.9
20.3
23.5
25.1
26.0
25.9
26.3
27.0
27.9
28.1
28.6
28.1
27.8
28.0
27.6
28.2
28.1
28.3
28.3
28.4
28.5
28.2
28.3
28.3
<==== Adjusted ===>
FTP
12.3
13.7
14.4
15.5
15.9
18.3
19.6
20.1
19.9
20.2
20.7
21.3
21.5
21.8
21.4
21.1
21.2
20.8
21.3
21.1
21.2
21.2
21.3
21.3
21.1
21.2
21.2
HFET
15.2
16.6
17.4
19.1
19.2
22.6
24.2
25.5
25.5
26.0
26.8
27.7
28.0
28.5
28.3
28.1
28.3
28.3
28.8
28.8
29.3
29.3
29.4
29.6
29.2
29.3
29.3
Composite*
13.5
14.9
15.6
16.9
17.2
20.0
21.4
22.2
22.1
22.4
23.0
23.8
24.0
24.4
24.0
23.7
23.9
23.6
24.1
24.0
24.2
24.2
24.3
24.4
24.1
24.2
24.2
* Composite fuel economies are a weighted harmonic average 55 percent FTP
and 45 percent HFET. They are referred to as "55/45" in Reference 1.
-------
-23-
DRAFT
Appendix C
Light-Truck Fuel Economy by Model Year
(Reproduced from Table 1 of Reference 1)
Model
Year
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
Lab
Composite
13.7
14.4
15.6
15.2
14.7
18.6
20.1
20.5
20.9
20.5
20.6
21.4
21.6
21.2
20.9
20.7
21.3
20.8
21.0
20.8
20.5
20.8
20.6
20.9
20.5
20.5
20.3
<==== Adjusted ===>
FTP
10.9
11.5
12.6
12.4
12.1
14.8
16.0
16.3
16.5
16.1
16.2
16.9
16.9
16.5
16.3
16.1
16.4
16.1
16.1
16.0
15.8
16.0
15.8
16.0
15.7
15.7
15.6
HFET
12.7
13.2
14.1
13.7
13.1
17.1
18.6
19.0
19.6
19.3
19.4
20.2
20.7
20.4
20.1
20.2
20.7
20.4
20.7
20.4
20.2
20.7
20.4
20.8
20.3
20.3
20.0
Composite
11.6
12.2
13.3
12.9
12.5
15.8
17.1
17.4
17.8
17.4
17.5
18.3
18.4
18.1
17.8
17.7
18.1
17.8
17.9
17.7
17.5
17.8
17.6
17.8
17.5
17.5
17.3
-------
-24-
DRAFT
Appendix D
Penetration of Diesels in the Light-Duty Fleet by Model Year
(Reproduced from Table 18 of Reference Number 6)
Model
Year
1974 and
Earlier
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996 and
Later
Passenger
Cars
0.67%
1.55%
1.37%
0.93%
1.14%
2.69%
3.90%
7.06%
5.10%
2.41%
1 .62%
0.97%
0.32%
0.27%
0.01%
0.04%
0.04%
0.13%
0.06%
0.03%
0.01%
0.06%
0.09%
Light-Dul
1 and 2
11.70%
10.38%
1.87%
0.00%
2.59%
3.16%
4.39%
6.16%
6.56%
2.23%
1 .20%
0.48%
0.33%
0.07%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
0.00%
ty Trucks
3 and 4
0.01%
0.00%
0.00%
0.00%
0.01%
0.11%
0.06%
0.13%
2.56%
2.09%
1 .69%
1.35%
1 .24%
0.82%
0.72%
0.83%
0.96%
1 .29%
1.15%
1 .45%
1.11%
1.15%
1 .26%
-------
-25-
DRAFT
Appendix E
Light-Duty Fuel Economy by Model Year
(Compiled from Tables 6 and 7 of Reference Number 2)
(Fuel Economy Values Are NOT Adjusted)
Model
Year
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
<= = = P
Gasoline
19.8
20.1
23.3
24.9
25.9
25.8
26.2
26.9
27.9
28.0
28.6
28.1
assenger C
Diesel
29.4
27.1
30.0
29.9
30.6
30.8
36.3
34.2
40.5
30.5
37.4
44.3
lars ===>
Combined
19.9
20.3
23.5
25.1
26.0
25.9
26.3
27.0
27.9
28.1
28.6
28.1
<= = =
Gasoline
15.2
14.7
18.4
19.7
20.0
20.7
20.4
20.5
21.4
21.6
21.1
20.9
Light Trucl
Diesel
21.2
21.1
24.3
32.0
27.0
27.0
27.4
26.1
26.7
25.6
22.2
22.6
Combined
15.2
14.7
18.6
20.1
20.5
20.9
20.5
20.6
21.4
21.6
21.2
20.9
-------
-26-
DRAFT
Appendix F
Merging Appendices B, C, and E
(ALL Fuel Economies Are Adjusted "Lab" 55/45)
Model
Year
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
<= = = P
Gasoline
16.8
17.1
19.8
21.2
22.0
21.9
22.2
22.9
23.7
23.8
24.3
23.9
assenger C
Diesel
25.0
23.0
25.5
25.4
26.0
26.2
30.9
29.1
34.4
25.9
31.8
37.7
lars ===>
Combined
13.5
14.9
15.6
16.9
17.2
20.0
21.4
22.2
22.1
22.4
23.0
23.8
24.0
24.4
24.0
23.7
23.9
23.6
24.1
24.0
24.2
24.2
24.3
24.4
24.1
24.2
24.2
<= = =
Gasoline
12.9
12.5
15.7
16.7
17.0
17.6
17.3
17.4
18.2
18.4
17.9
17.8
Light Trucl
Diesel
18.0
17.9
20.7
27.2
23.0
23.0
23.3
22.2
22.7
21.8
18.9
19.2
Combined
11.6
12.2
13.3
12.9
12.5
15.8
17.1
17.4
17.8
17.4
17.5
18.3
18.4
18.1
17.8
17.7
18.1
17.8
17.9
17.7
17.5
17.8
17.6
17.8
17.5
17.5
17.3
-------
-27-
DRAFT
Appendix G
Estimating Missing Values for Appendix F
Model
Year
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
<= = = P
Gasoline
13.4
14.8
15.5
16.8
17.1
19.8
21.2
22.0
21.9
22.2
22.9
23.7
23.8
24.3
23.9
23.6
23.8
23.5
24.0
23.9
24.t
24.t
24.2
24.3
24.0
24.t
24.t
assenger C
Diesel
24.0
24.0
24.0
25.0
23.0
25.5
25.4
26.0
26.2
30.9
29.1
34.4
25.9
31.8
37.7
32.4
32.4
32.4
32.4
32.4
32.4
32.4
32.4
32.4
32.4
32.4
32.4
lars ===>
Combined
13.5
14.9
15.6
16.9
17.2
20.0
21.4
22.2
22.1
22.4
23.0
23.8
24.0
24.4
24.0
23.7
23.9
23.6
24.1
24.0
24.2
24.2
24.3
24.4
24.1
24.2
24.2
<= = =
Gasoline
11.2
12.1
13.2
12.9
12.5
15.7
16.7
17.0
17.6
17.3
17.4
18.2
18.4
17.9
17.8
17.6
18.0
17.7
17.8
17.6
17.4
17.7
17.5
17.7
17.4
17.4
17.2
Light Trucl
Diesel
18.0
18.0
18.0
18.0
17.9
20.7
27.2
23.0
23.0
23.3
22.2
22.7
21.8
18.9
19.2
20.6
20.6
20.6
20.6
20.6
20.6
20.6
20.6
20.6
20.6
20.6
20.6
Combined
11.6
12.2
13.3
12.9
12.5
15.8
17.1
17.4
17.8
17.4
17.5
18.3
18.4
18.1
17.8
17.7
18.1
17.8
17.9
17.7
17.5
17.8
17.6
17.8
17.5
17.5
17.3
-------
-28-
DRAFT
Appendix H
Expanding Fuel Economy Estimates
From Appendix G to ALL Light-Duty Classes
Model
Year
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
<=== Gasoline- Fueled ===>
LDGV
13.4
14.8
15.5
16.8
17.1
19.8
21.2
22.0
21.9
22.2
22.9
23.7
23.8
24.3
23.9
23.6
23.8
23.5
24.0
23.9
24.1
24.1
24.2
24.3
24.0
24.1
24.1
LDGT12
12.0
13.0
14.2
13.9
13.4
16.8
18.0
18.3
18.9
18.6
18.7
19.6
19.7
19.3
19.1
18.9
19.4
19.0
19.1
18.9
18.7
19.0
18.8
19.0
18.7
18.7
18.5
LDGT34
9.3
10.0
10.9
10.7
10.3
12.9
13.8
14.1
14.5
14.3
14.4
15.0
15.2
14.8
14.7
14.6
14.9
14.6
14.7
14.6
14.4
14.6
14.5
14.6
14.4
14.4
14.2
<=== Diesel-Fueled ===>
LDDV
24.0
24.0
24.0
25.0
23.0
25.5
25.4
26.0
26.2
30.9
29.1
34.4
25.9
31.8
37.7
34.2
34.2
34.2
34.2
34.2
34.2
34.2
34.2
34.2
34.2
34.2
34.2
LDDT12
19.4
19.4
19.4
19.4
19.3
22.2
29.2
24.7
24.7
25.0
23.8
24.4
23.4
20.3
20.7
22.1
22.1
22.1
22.1
22.1
22.1
22.1
22.1
22.1
22.1
22.1
22.1
LDDT34
14.9
14.9
14.9
14.9
14.8
17.1
22.5
19.0
19.0
19.3
18.3
18.8
18.0
15.6
15.9
17.0
17.0
17.0
17.0
17.0
17.0
17.0
17.0
17.0
17.0
17.0
17.0
-------
-29-
DRAFT
Appendix I
TIUS Fuel Economy Survey Data on Which Tables 2 and 3 Are Based
(from Reference 3, Tables A-1 through A-15)
Summarized Gasoline Heavy-Duty Vehicle Fuel Economies (mpg)
Model
Year
83
84
85
86
87
88
89
90
91
92
Weight Classes:
2b
9.2
8.6
9.0
8.7
9.4
9.7
9.4
9.7
8.7
10.4
3
7.1
8.4
8.3
8.2
9.1
8.8
10.2
9.6
8.9
10.7
4
6.5
6.3
8.1
7.7
7.9
8.1
9.0
9.9
9.5
8.9
5
6.7
8.2
7.0
7.3
8.4
7.2
6.3
8.5
7.6
7.3
6
6.3
7.2
7.3
7.5
6.8
7.9
6.9
7.7
8.8
7.6
7
7.6
6.0
6.7
6.6
6.1
7.7
5.7
7.7
7.7
13.8
8A
4.5
8.5
5.8
6.4
8.5
5.6
5.1
7.2
7.8
6.3
8B
Summarized Diesel Heavy-Duty Vehicle Fuel Economies (mpg)
Model
Year
83
84
85
86
87
88
89
90
91
92
Weight Classes:
2b
10.8
13.2
11.3
12.2
11.7
11.8
12.4
11.6
12.0
12.4
3
9.8
9.6
11.0
11.9
10.6
11.8
10.6
11.6
11.2
11.1
4
8.1
6.4
8.3
9.0
9.5
9.7
10.4
10.1
12.9
9.9
5
7.6
8.1
9.9
8.4
9.2
9.9
10.7
9.2
9.4
9.7
6
8.0
8.0
7.6
8.1
8.3
8.9
8.8
8.1
8.2
8.3
7
5.9
7.0
7.7
7.3
7.6
8.0
7.2
7.4
7.3
7.5
8A
5.5
5.7
5.9
6.0
6.1
6.1
5.9
6.1
6.4
6.2
8B
5.2
5.3
5.3
5.5
5.5
5.5
5.7
5.8
5.9
6.0
-------
-30- DRAFT
Appendix J
Response to Peer Review Comments from Christian Lindhjem, ENVIRON Corp.
An earlier version of this report was formally peer reviewed
by two peer reviewers (Christian Lindhjem and Marc Ross). In
this appendix, comments from Christian Lindhjem are reproduced in
plain text, and EPA's responses to those comments are
interspersed in indented italics. Comments from the other peer
reviewer appear in the following appendix (Appendix K).
************************************
ENVIRON Review of
EPA Draft Report:
"Updating Fuel Economy Estimates in MOBILE6.3"
Prepared for
EPA
Office of Transportation and Air Quality
2000 Traverwood Drive
Ann Arbor, MI 48105-2425
Prepared by
Christian Lindhjem
ENVIRON International Corporation
101 Rowland Way, Suite 220
Novato, CA 94945
10 June 2002
Introduction
Overall, this report reasonably details the conventional understanding of the in-use fuel economy
for current and past model year vehicles. While these estimates may indeed be accurate within
the scope and intent of this model, there are clear limitations with these estimates. Therefore, the
intended use of MOBILE6.3 needs to be clearly stated and consistent with the simplifying
assumptions used in these estimates. The estimates provided in this work only sought to
distinguish vehicle fuel consumption by model year and general vehicle types such as diesel and
gasoline light-duty cars, 2 kinds of pick-up trucks, and heavy-duty vehicles of various gross
vehicle weight ratings (GVWR). All other differences that could affect fuel economy were
ignored limiting the model's use to just these variables.
-------
-31- DRAFT
RESPONSE: The Users Guide for MOBILE6.3 (which is being drafted)
will stress these limitations.
For light-duty vehicle, the current fuel economy information gathered through the certification
procedure is the ultimate source of data used to determine fuel economy distinguishing only by
vehicle type and model year. One in-use adjustment was given to all vehicles ignoring fuel
economy improvements that might be experienced in use by technology type, model year, or as
vehicles age. This limits the analysis to differences experienced only during the certification test
procedure when the vehicles are first certified.
For heavy-duty vehicles, the current fuel economy information is based on, what is now called,
the VIUS surveys (http://www.census.gov/svsd/www/tiusview.html). Because the fuel economy
was determined from in-use surveys, no in-use adjustment was needed. As with the light-duty
estimates, the heavy-duty estimates only distinguish by vehicle type and model year.
RESPONSE: A statement to that effect has been added to
Section 2.2.
Specific Issues
On page 3, the light-duty adjustment was not well described and needs to be explained in greater
detail. Searching the referenced source, the most recent Fuel Economy Trends report, did not
provide a clear explanation of these adjustments. An early report (SAE-820791) described a
statistical regression of in-use survey data and distinguished the adjustment through the variables
of number of cylinders, front-wheeled drive, manual transmission, fuel injection, and diesel.
Could these variables have been used to determine the adjustment by technology type and
combined with sales data could have provided unique adjustments by model year? The in-use
adjustment therefore needs to be discussed in greater detail especially in regards to the effect that
model year or technology type might have in the estimates used in the model. MOBILE6
described an improvement of the emission deterioration with newer model year vehicles, so it
would be expected that such reduced deterioration might lead to an improvement in in-use fuel
consumption performance as well.
RESPONSE: It is true that a number of different approaches for
creating adjustment factors are possible. At the time
of that SAE paper (around 1982), EPA considered the
same approaches identified by this reviewer to adjust
its official fuel economy estimates. EPA eventually
settled on the approach used in this report. Since EPA
began adjusting the dynamometer test results to better
approximate on-road fuel economies (beginning with the
1985 model year), the ONLY adjustment factors used by
EPA have been the reduction of the FTP fuel economy
values by 10 percent and the reduction of the HFET
values by 22 percent (effectively reducing the
composite values by about 15 percent). EPA will
continue to use these historic adjustment factors in
this interim model. At the suggestion of the reviewer,
-------
-32- DRAFT
we have expanded the discussion (on page 3) of these
adjustment factors.
Also on page 3, it is stated that the relative mileage on the city and highway driving cycles, the
55/45 composite value, is an appropriate estimate of real world (in-use) conditions. EPA should
either justify this claim through national VMT estimates, determining whether such an estimate
is accurately historically, or a discussion needs to be added to include this estimate to the list of
limitations of MOBILE6.3.
RESPONSE: As with the previous point, the 55/45 (FTP/HFET) split
is EPA's official (and historic) approach to simulating
actual vehicle usage. It is used in EPA's Gas Guzzler
and CAFE programs. Also, this composite value (after
"adjusting") is used in EPA's fuel economy labeling
program for new light-duty vehicles. Therefore, the
use of this weighted composite value to represent real-
world driving has already been discussed.
On page 6, it is stated that in-use vehicle weight data for light-duty trucks was taken from the
Mobile Source Observational Database (MSOD) to allow for distinguishing between LDT12
from LDT34. Any data set should be accompanied by a discussion of the potential selection bias,
though in this case the vehicle weights were only used to determine a ratio of curb weights so
selection bias may be masked. However for instance, one could envision a circumstance where
LDT34 may have higher in-use curb weights compared with LDT12 than in the data set because
of selection bias by choosing fewer commercially licensed work trucks or removal of tools or
other loads prior to testing for the MSOD.
RESPONSE: The data in the MSOD was gathered for dozens of
individual work assignments. Therefore, while the
vehicle selection criteria of an individual work
assignment might have been skewed (i.e., biased), we
believe that combined vehicle recruitment was most
likely representative. However, the reviewer is
correct in that none of the test results included the
effects of cargo (including work tools). Including
tools, passengers, and other cargo would increase the
weight of the vehicle above the dynamometer test weight
(possibly as high as the GVWR) . This increase in
vehicle weight would result in a decrease in fuel
economy and may be one of the reasons that those EPA
fuel economy "adjustment factors" are necessary.
On page 8 with the table on page 9, no discussion of the effect of the heavy-duty defeat device is
provided even though this issue figured prominently in MOBILE6. While it may be difficult to
assess the fuel economy effect of the defeat device, the lack of discussion begs the question, so a
discussion needs to be included.
RESPONSE: A discussion has been added to the top of page 10 on
this question. Additionally, the raw fuel economy
values (in Appendix I) indicate that the increases in
-------
-33-
DRAFT
the fuel economy of the diesel-fueled heavy-duty trucks
parallel the increases of the corresponding gasoline-
fueled heavy-duty trucks. This suggests that the fuel
economy improvements of those diesel-fueled heavy-duty
trucks may result from improvements to tires and
aerodynamics rather than to alleged NOx defeat devices.
On page 14, when estimating the total fuel consumption, the discussion of the vehicle miles
traveled (VMT) needs to be accompanied by a discussion of how the vehicle mix (fraction of
VMT by vehicle type) and the diesel/gasoline fractions were developed.
RESPONSE: The approach was revised to simply use the MOBILE6 VMT
fractions along with the total fleet VMT. The footnote
to Table 9 was added to explain this.
On page 16, the fuel consumption estimates available from EIA (http://www.eia.doe.gov/), could
also be used to compare with the estimates provided here. Based on the EIA estimates, a
comparison with the estimates in Table 10 can be made and is shown in Table 1. There may be a
number of reasons why the EIA estimates are higher than those predicted by MOBILE6.3
including the use of highway fuel in nonroad engines, spillage, or other losses or uses.
Table 1. Comparison of 1998 CO2 emissions (millions of metric tons).
GI
Gasoline
1,048
IG
Diesel
247
1,295
MOBILE6
Gasoline
991
.3 Estimates
Diesel
274
1,265
EIAEs
Gasoline
1,124
timates
Diesel
310
1,434
RESPONSE: As the reviewer suggested, Table 11 was added to
include the fuel consumption estimates of the Energy
Information Administration (EIA).
Discussion and Recommendation for Future Work
This report could provide a blueprint for future work suggesting issues that need to be
investigated. Future work needs to address many factors, but the most important estimate in the
current work that needs to be investigated more thoroughly is the in-use adjustment discussed in
detail in SAE-820791 and shown below. Vehicle emission standards (such as improved cold
start controls from emission standards, off-cycle testing procedures, evaporative controls), in-use
programs (inspection and maintenance or on-board diagnostics), or other programs may affect the
in-use adjustment to light-duty fuel economy.
RESPONSE: It is anticipated that future models will rely heavily
on actual in-use test results. Therefore, these
adjustment factors may not be necessary in the future
models.
-------
-34- DRAFT
Factors Affecting Fuel Economy (from SAE-820791)
Production Slip
Administrative variance
Hardware variance
Vehicle Condition
Engine tune
Engine response to fuel properties
Sampling bias
Travel Environment
Ambient temperature
Barometric Pressure/Altitude
Wind and Aerodynamics
Road Gradient
Road Surface
Road Curvature
Travel Characteristic
Vehicle speed
Traffic volume effects
Trip length/Vehicle warm up
Acceleration intensity
Vehicle Condition
Wheel mechanical condition
Tire pressure
Vehicle weight load
The effect on fuel economy of each of these factors could be influenced by the technology type,
model year, or maintenance condition of vehicles. The current estimates of the in-use adjustment
in this draft EPA report ignore these factors.
RESPONSE: These factors were intentionally ignored for this
interim model. They will be considered in future
models.
In addition, there was no discussion of CNG/LPG or other alternatively fueled vehicles.
MOBILE6 has included emission estimates for these vehicles; so it may be important to include
such estimates in MOBILE6.3 raise questions (sic). Applications that have begun to use CNG
(or LNG) and LPG-fueled vehicles included many light-duty vehicles and transit buses, refuse
trucks, and other heavy-duty vehicles, so a comparison of such technologies might be a useful
purpose for MOBILE6.3.
-------
-35- DRAFT
RESPONSE: The fuel economies of vehicles other than diesel-fueled
or gasoline-fueled are outside the scope of MOBILE6.3.
They will be considered when the analyses for the New
Generation GHG model are performed.
Also, fuel differences, such as summer/winter, reformulated or oxygenated gasoline and diesel,
were not discussed. The energy content of these fuels may affect the overall estimates of fuel
consumption in mile per gallon. A consideration of these effects need to included if the
predicted fuel consumption is to be compared with fuel sales or production.
RESPONSE: The effects of those parameters are outside the scope
of MOBILE6.3. They will be considered when the
analyses for the New Generation GHG model are
performed.
It is expected that the future editions of the MOBILE6.3 model would include the technology and
analysis being developed for future regulated emissions modeling (such as described at
http://www.epa.gov/otaq/ngm.htm). It would be helpful therefore to describe how new
emissions/fuel consumption data will be used to improve the fuel consumption estimates
provided here.
RESPONSE: After MOBILE6.3 is completed, we do not plan to prepare
additional "editions." Instead, our efforts will focus
on the New Generation GHG model. However, the User's
Guide for MOBILE6.3 will explain how to alter the
default fuel economy estimates so that the users may
model various future scenarios.
When making comparisons with fuel consumption predictions, it is useful to determine the
uncertainty of the estimates provided. It is particularly important for MOBILE6.3 where many of
the estimates are based on proprietary information, such as the confidential business information
of sales figures, which prevent independent assessment of the results provided in this report. An
uncertainty analysis is important not only for assessing the predictions made by the model but in
improving the estimates by targeting future work toward estimates with the greatest uncertainty.
RESPONSE: Analyses of uncertainty have not been performed for any
of the versions of MOBILE6. A limited discussion of
the uncertainty (qualitative not quantitative) in the
total fleet C02 emissions and fuel consumption is
included on page 19. The question of uncertainty will
be considered when the analyses for the New Generation
GHG model are performed.
-------
-36- DRAFT
Appendix J-1
Appendix (to Peer Reviewer's Report)
Editorial/Typographical Errors
On page 7, there was a typo where the reference "Tables 3 and 4" should be "Tables 2 & 3".
RESPONSE: The text has been corrected.
On the top of page 10, there is a description of the trends shown in Tables 2 and 3, but these
trends were smoothed regressions of the TIUS results. Therefore, it is not surprising that these
trends would be consistent.
RESPONSE: The text has been revised, and Appendix I (which
contains the TIUS "raw" fuel economies) has been added.
-------
-37- DRAFT
Appendix K
Response to Peer Review Comments from Marc Ross, University of Michigan
This report was formally peer reviewed by two peer reviewers
(Christian Lindhjem and Marc Ross). In this appendix, comments
from Marc Ross are reproduced in plain text, and EPA's responses
to those comments are interspersed in indented italics. Comments
from the other peer reviewer appear in the preceding appendix
(Appendix J).
************************************
Review of "Updating Fuel Economy Estimates in MOBILE6.3",
by Larry C. Landman, March 26, 2002
Suggestions
p 3 The Hellman-Murrell fuel economy adjustment is still in use,
but one may question whether the factors are still valid after
almost 20 years with their changes in vehicles, roads and speeds.
We don't have anything that's better, so I suggest adding cites,
but not changing the calculation. One cite is Mintz, Marianne,
Anant Vyas, and L. A. Conley 1993, "Differences Between EPA-test
and In-use Fuel Economy: Are the Correction Factors Correct?"
Paper No. 931104, Transportation Research Board, Washington, DC.
RESPONSE: A footnote to that effect has been added to the bottom
of page 3. Also, it is anticipated that future models
will rely heavily on actual in-use test results. Thus,
these adjustment factors may not be necessary in future
models.
p 6 The fraction of LDT12 among LDT1234, and the ratio of fuel
economies for them are a bit troubling. In the Fleet
Characterization Data for MOBILE6 (Jackson) Table 10, it says
MOBILE6 LDT3 has GVWR >6000 Ibs and includes most 1/2 ton pickups
(others being LDT4). But Buyers' Guides shows popular 1/2 ton
pickups with GVW (curb weight plus maximum payload) under 5000
Ibs for Ranger and S-10 in 1991; and 1/2 ton full size pickups
are just under and over 6000 Ibs. So I find the sales fraction
for LDT12 may be > 2/3. Moreover Jackson Table 13, shows 74% for
LDT12 among LDT1234 for 1996, which may have been the data year
for that analysis.
RESPONSE: The reviewer is correct; the actual ratio should be
about three-fourths (not two-thirds). The text has
been corrected. The values in Section 4.0 and in
Appendix H have been recalculated.
-------
-38- DRAFT
In the same vein, if I look at the fuel economies of pickups, I
see a big difference between Ranger and S-10 on the one hand, and
the 3/4 and 1 ton pickups on the other.
RESPONSE: This is not unexpected since the fuel economies within
each vehicle class can vary greatly.
In addition, the footnote on page 6 is ambiguous as it talks
about the ratio of LDT12 to LDT34 where presumably it's LDT12 to
LDT1234.
RESPONSE: That footnote has been corrected.
I haven't done any analysis here, just looked at a few examples
among pickups including a look at sales in 1990; and have not
considered vans.
p!4 The unstated assumption in the comparisons of fuel
consumption based on DOT and MOBILE6 is that vehicle fuel economy
doesn't change over the life of the vehicle. (Since the DOT data
is based on vehicles of all ages and the EPA data on new
vehicles.)
RESPONSE: A statement to that effect has been added just prior to
Section 2.1.1.
One would expect some change in fuel economy with vehicle age,
for example as tire pressure and tire rolling resistance change,
engine friction changes, and possibly an external rack is added.
A cite is Ang, B.W., T.F. Fwa, et al. "A Statistical Study on
Automobile Fuel Consumption", Energy vol 16, no. 1, pp!067-1077,
1991.
On the other hand, I once looked at C02 emissions by odometer
reading in the FTP surveys by EPA and could not find an effect.
RESPONSE: Our assumption that fuel economy does not change as
vehicles age is controversial and probably overly
simplistic. In future models, vehicle age (as well as
other variables) will be considered.
Other Comments
p2 The motivations for the work are well explained, i.e. the need
to separate diesel from gasoline-fueled vehicles, the need for
documentation, and the decision not to include local or regional
differences in driving. The need to definitively separate diesel
from gasoline-fueled vehicles for carbon analysis could be stated
explicitly.
RESPONSE: A statement to that effect has been added to the
beginning of Section 2.1.1.
-------
-39- DRAFT
pp 4,5 The decisions made on diesel penetrations and fuel
economies are well justified by the small number of diesels at
issue - that were produced or that are still on the road.
pp 7,8,9 It appears at first glance that the ratios of the
coefficients, B/A, vary too much for one to be comfortable
extrapolating them into the future, unless one had information on
differences in model changes. (Compare, for example, HDV3 with
HDV7.) However, the time dependence of the B terms is slight.
Thus for HDV3 the fuel economy ratio for MY96 to MY92 is 1.046,
while that for HDV7 is 1.007, a difference of only 4%.
RESPONSE: The reviewer is correct that the fuel economy changes
only very slightly with time (i.e., model year). These
regression equations were used (in Reference Number 3)
to extrapolate the fuel economies only for a relatively
short time into the future (i.e., four years).
pp 10,11 It is a good choice to focus the classification on
school buses as done here.
p 12 The last comments before Table 6 are wonderful.
p 13 The decision about expanding the model year range into the
past is good. One can only hope that the extension into the
distant future will prove highly inaccurate, but that
applications of MOBILE6.3 will, at most, involve model years only
a few years beyond the present.
p 14 In the first sentence below Table 8 it says "four light-
duty" truck classes, but Appendix A has 4 LDGT and 2 LDDT listed.
RESPONSE: In this report, the LDGT1 and LDGT2 were combined into
a single class (LDGT12) as were the LDGT3 and LDGT4
(forming LDGT34). A footnote was added explaining
this.
p 17 The most critical statement is made just below Fig. 2. The
VMT estimates are crude.
RESPONSE: The combined fleet estimates are very sensitive to VMT
which is itself difficult to estimate.
Appendices The tables are clear and easy to understand. The
table number from which the data was taken is not shown for App
E.
RESPONSE: The data in Appendix E were compiled from values in
Tables 6 and 7 of the 1989 "Trends" report. Those
table numbers have now been added to Appendix E.
------- |