United States

Environmental Protection

Agency

Air and Radiation EPA420-P-02-005

August 2002

M6.GHG.001

Updating Fuel Economy

Estimates in MOBILE6.3

 Printed on Recycled Paper

DRAFT
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

CO2 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.
*** DRAFT ***

Updating Fuel Economy Estimates in MOBILE6

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 (CO2), air

conditioning refrigerants, nitrous oxide (N2O), 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 CO2

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. CO2

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 CO2.*

________________________________

* EPA calculates fuel consumption (gallons per mile) as a linear combination

of HC, CO, and CO2 (see 40 CFR 600.113). Therefore, any one of those four

values (i.e., CO2) 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 CO2 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 CO2

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 CO2 emissions for the gasolinefueled 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 lightduty 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 gasolinefueled 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 threefourths 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 MOBILE6 (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

Gasoline-Fueled Diesel-Fueled

Class A B A B

HDV2b 0.1253 0.9624 0.1072 1.0506

HDV3 0.1157 0.9632 0.0989 1.0450

HDV4 0.0409 1.1902 0.5020 0.6598

HDV5 0.4416 0.6348 0.2474 0.8078

HDV6 0.0338 1.2015 0.5336 0.6117

HDV7 0.1277 0.8909 4.0206 0.1374

HDV8a 0.0647 1.0285 0.15485 0.8194

HDV8b --- --- 0.0119 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 HDGV8b trucks are rare or non-existent, and MOBILE6 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 Weight Classes:

Year 2b_ 3 4 5 6 7 8A 8B

83 8.81 8.16 7.87 7.30 6.83 6.54 6.09 5.43

84 8.91 8.26 7.98 7.35 6.93 6.62 6.17 5.55

85 9.01 8.35 8.09 7.41 7.03 6.69 6.24 5.64

86 9.11 8.45 8.21 7.47 7.13 6.76 6.32 5.75

87 9.22 8.54 8.32 7.52 7.23 6.83 6.39 5.91

88 9.32 8.63 8.43 7.58 7.33 6.89 6.47 6.00

89 9.42 8.73 8.55 7.63 7.43 6.96 6.54 6.09

90 9.52 8.82 8.66 7.68 7.53 7.03 6.62 6.19

91 9.62 8.92 8.78 7.74 7.63 7.10 6.70 6.29

92 9.73 9.01 8.89 7.79 7.73 7.17 6.77 6.38

93 9.83 9.11 9.01 7.85 7.84 7.24 6.85 6.47

94 9.93 9.20 9.12 7.90 7.94 7.31 6.92 6.57

95 10.03 9.30 9.24 7.95 8.04 7.38 7.00 6.67

96 10.13 9.39 9.35 8.01 8.14 7.45 7.07 6.76

Table 3

Projected Diesel Heavy-Duty Vehicle Fuel Economies (mpg)

Model Weight Classes:

Year 2b_ 3 4 5 6 7 8A 8B

83 11.13 10.01 9.27 8.78 7.96 7.38 5.79 5.16

84 11.27 10.14 9.34 8.87 8.02 7.39 5.84 5.25

85 11.41 10.27 9.41 8.95 8.08 7.40 5.90 5.33

86 11.55 10.39 9.49 9.04 8.14 7.41 5.96 5.42

87 11.69 10.52 9.56 9.12 8.20 7.43 5.96 5.51

88 11.83 10.65 9.63 9.21 8.25 7.44 6.03 5.59

89 11.97 10.77 9.70 9.29 8.31 7.45 6.10 5.68

90 12.11 10.90 9.77 9.38 8.37 7.46 6.17 5.77

91 12.26 11.03 9.85 9.46 8.42 7.47 6.24 5.86

92 12.40 11.15 9.92 9.54 8.48 7.48 6.31 5.95

93 12.54 11.28 9.99 9.63 8.54 7.49 6.38 6.03

94 12.68 11.41 10.06 9.71 8.59 7.51 6.45 6.12

95 12.82 11.53 10.13 9.80 8.65 7.52 6.52 6.21

96 12.96 11.66 10.20 9.88 8.71 7.53 6.59 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 <=== Diesel-Fueled ===> <=== Gasoline-Fueled ===>

Year Transit Intercity School Transit Intercity School

1987 3.43 4.64 6.29 3.11 3.64 6.18

1988 3.47 4.69 6.28 3.15 3.68 6.21

1989 3.51 4.75 6.27 3.19 3.72 6.24

1990 3.55 4.80 6.25 3.22 3.76 6.27

1991 3.59 4.85 6.24 3.26 3.80 6.30

1992 3.63 4.91 6.23 3.30 3.85 6.33

1993 3.67 4.96 6.22 3.33 3.89 6.37

1994 3.71 5.01 6.20 3.37 3.93 6.40

1995 3.75 5.07 6.19 3.40 3.97 6.42

1996 3.79 5.12 6.18 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), and

 -- HDDBS (diesel-fueled school buses).

Therefore, we must combine the two transit and urban dieselfueled 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 MOBILE6 classes)

Model

Year HDDBT HDDBS HDGB

1987 3.94 6.29 6.18

1988 3.99 6.28 6.21

1989 4.04 6.27 6.24

1990 4.08 6.25 6.27

1991 4.13 6.24 6.30

1992 4.17 6.23 6.33

1993 4.22 6.22 6.37

1994 4.26 6.20 6.40

1995 4.31 6.19 6.42

1996 4.36 6.18 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 onroad motorcycles averaged 50.0 mpg for each model year.
-13- DRAFT

Table 6

DOT Estimates of Fuel Consumption

of the In-Use Motorcycle Fleet

Gasoline Calculated

VMT Consumed Fleet Fuel

Calendar (millions (millions of Economy

Year of miles) gallons) (mpg)

1970 3,000 60 50.00

1975 5,600 113 49.56

1980 10,200 204 50.00

1985 9,100 182 50.00

1990 9,600 191 50.26

1991 9,200 184 50.00

1992 9,600 191 50.26

1993 9,900 198 50.00

1994 10,200 205 49.76

1995 9,800 196 50.00

1996 9,900 198 50.00

1997 10,100 202 50.00

1998 10,300 205 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 offroad) 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

VMT

(millions

of miles)

Fuel

(millions of

gallons)

Passenger cars 1,545,830 72,209

Motorcycles 10,260 205

Other Vehicle s with 2-Axles

and 4-Tires

866,228 50,579

Single-Unit Trucks with 2-Axles

and 6 or More Tires

67,894 9,741

Combination Trucks 128,159 21,100

Bus 6,996 1,049

Totals: 2,625,367 154,883
-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

DOT Estimates ---------- MOBILE6.3 Estimates ----------

Vehicle Types

VMT

(millions

of miles)

Fuel Used

(millions of

gallons)

VMT

Fractions

VMT**

(millions

of miles)

Fuel

Economies

(mpg)

Fuel Used

(millions

of gallons)

Passenger Cars 1,545,830 72,209 52.54% 1,379,298 23.41 58,922

Motorcycles 10,260 205 0.65% 17,130 50.00 343

ALL Trucks 1,062,281 81,420 46.53% 1,221,588 13.16 92,809

Bus 6,996 1,049 0.28% 7,351 5.32 1,381

Totals: 2,625,367 154,883 2,625,367 153,455

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. Similarly, we combined

the LDGT3 and LDGT4 to form the LDGT34.

** 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

 0

 40

 80

 120

 160

T otal AL L T rucks Cars B us Motorcycle

 DOE E stimates

 MOB IL E 6.3

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 CO2 emission factors.

Using these CO2 emission factors and the estimated VMTs from

Table 9, we calculated the CO2 emissions of the on-road (highway)

fleet. Those estimates of fleet CO2 emissions are given in Table

10. For comparison, Table 10 includes "top down" estimates of

CO2 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)

From GHG Report MOBILE6.3 Estimates

Vehicle Types

CO2 from

GasolineFueled Veh.

CO2 from

DieselFueled Veh.

CO2 from

GasolineFueled Veh.

CO2 from

DieselFueled Veh.

Passenger Cars 670.16 7.69 491.59 1.71

Motorcycles 1.83 --- 2.50 ---

ALL Trucks 373.74 229.74 520.45 299.51

Bus 2.20 9.16 2.10 10.92

Totals: 1,047.93 246.59 1,016.64 312.14

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 CO2 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 CO2 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 CO2 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 CO2 emissions is

from the Energy Information Administration (EIA)* of the US

Department of Energy. EIA's estimates of CO2 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 CO2 Emission

By Vehicle Type for 1998

(millions of metric tons)

Source of Estimates

CO2 from

GasolineFueled Veh.

CO2 from

DieselFueled Veh. TOTAL

EIA 1,124 310 1,434

MOBILE6.3 1,017 312 1,329

EPA's GHG Report 1,048 247 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)

0

500

1,000

1,500

CO2 from GasolineF ueled Veh.CO2 from D ieselF ueled Veh.T otal CO2

E IA

MOB IL E 6.3

E P A's GH G R eport

From the data in Table 11 (or from Figure 2), we observe:

 - The MOBILE6.3 prediction of total CO2 emissions (from diesel

plus gasoline) is between the estimates for the two "top

down" models of CO2 emissions.
-19- DRAFT

 - The MOBILE6.3 prediction of CO2 emissions from gasolinefueled vehicles is less than the estimates for the two "top

down" models of CO2 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 CO2 emissions from diesel-fueled

vehicles is more than the estimates for the two "top down"

models of CO2 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 CO2 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 EPATest 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.htm#msod)

 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 MOBILE6

 No. Abbrev. Description

 1 LDGV Light-Duty Gasoline Vehicles (Passenger Cars)

 2 LDGT1 Light-Duty Gasoline Trucks 1 (0-6,000 lbs. GVWR, 0-3,750 lbs.

LVW)

 3 LDGT2 Light-Duty Gasoline Trucks 2 (0-6,000 lbs. GVWR, 3,751-5750 lbs.

LVW)

 4 LDGT3 Light-Duty Gasoline Trucks 3 (6,001-8500 lbs. GVWR, 0-3750 lbs.

LVW)

 5 LDGT4 Light-Duty Gasoline Trucks 4 (6,001-8500 lbs. GVWR, 3,751-5750

lbs. LVW)

 6 HDGV2b Class 2b Heavy-Duty Gasoline Vehicles (8501-10,000 lbs. GVWR)

 7 HDGV3 Class 3 Heavy-Duty Gasoline Vehicles (10,001-14,000 lbs.

GVWR)

 8 HDGV4 Class 4 Heavy-Duty Gasoline Vehicles (14,001-16,000 lbs.

GVWR)

 9 HDGV5 Class 5 Heavy-Duty Gasoline Vehicles (16,001-19,500 lbs.

GVWR)

10 HDGV6 Class 6 Heavy-Duty Gasoline Vehicles (19,501-26,000 lbs.

GVWR)

11 HDGV7 Class 7 Heavy-Duty Gasoline Vehicles (26,001-33,000 lbs.

GVWR)

12 HDGV8a Class 8a Heavy-Duty Gasoline Vehicles (33,001-60,000 lbs.

GVWR)

13 HDGV8b Class 8b Heavy-Duty Gasoline Vehicles (Over 60,000 lbs. GVWR)

14 LDDV Light-Duty Diesel Vehicles (Passenger Cars)

15 LDDT12 Light-Duty Diesel Trucks 1and 2 (0-6,000 lbs. GVWR)

16 HDDV2b Class 2b Heavy-Duty Diesel Vehicles (8501-10,000 lbs. GVWR)

17 HDDV3 Class 3 Heavy-Duty Diesel Vehicles (10,001-14,000 lbs. GVWR)

18 HDDV4 Class 4 Heavy-Duty Diesel Vehicles (14,001-16,000 lbs. GVWR)

19 HDDV5 Class 5 Heavy-Duty Diesel Vehicles (16,001-19,500 lbs. GVWR)

20 HDDV6 Class 6 Heavy-Duty Diesel Vehicles (19,501-26,000 lbs. GVWR)

21 HDDV7 Class 7 Heavy-Duty Diesel Vehicles (26,001-33,000 lbs. GVWR)

22 HDDV8a Class 8a Heavy-Duty Diesel Vehicles (33,001-60,000 lbs. GVWR)

23 HDDV8b Class 8b Heavy-Duty Diesel Vehicles (Over 60,000 lbs. GVWR)

24 MC Motorcycles (Gasoline)

25 HDGB Gasoline Buses (School, Transit and Urban)

26 HDDBT Diesel Transit and Urban Buses

27 HDDBS Diesel School Buses

28 LDDT34 Light-Duty Diesel Trucks 3 and 4 (6,001-8,500 lbs. GVWR)
-22- DRAFT

Appendix B

Passenger Car Fuel Economy by Model Year

(Reproduced from Table 1 of Reference 1)

Model Lab <==== Adjusted ===>

Year Composite* FTP HFET Composite*

1975 15.8 12.3 15.2 13.5

1976 17.5 13.7 16.6 14.9

1977 18.3 14.4 17.4 15.6

1978 19.9 15.5 19.1 16.9

1979 20.3 15.9 19.2 17.2

1980 23.5 18.3 22.6 20.0

1981 25.1 19.6 24.2 21.4

1982 26.0 20.1 25.5 22.2

1983 25.9 19.9 25.5 22.1

1984 26.3 20.2 26.0 22.4

1985 27.0 20.7 26.8 23.0

1986 27.9 21.3 27.7 23.8

1987 28.1 21.5 28.0 24.0

1988 28.6 21.8 28.5 24.4

1989 28.1 21.4 28.3 24.0

1990 27.8 21.1 28.1 23.7

1991 28.0 21.2 28.3 23.9

1992 27.6 20.8 28.3 23.6

1993 28.2 21.3 28.8 24.1

1994 28.1 21.1 28.8 24.0

1995 28.3 21.2 29.3 24.2

1996 28.3 21.2 29.3 24.2

1997 28.4 21.3 29.4 24.3

1998 28.5 21.3 29.6 24.4

1999 28.2 21.1 29.2 24.1

2000 28.3 21.2 29.3 24.2

2001 28.3 21.2 29.3 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 Lab <==== Adjusted ===>

Year Composite FTP HFET Composite

1975 13.7 10.9 12.7 11.6

1976 14.4 11.5 13.2 12.2

1977 15.6 12.6 14.1 13.3

1978 15.2 12.4 13.7 12.9

1979 14.7 12.1 13.1 12.5

1980 18.6 14.8 17.1 15.8

1981 20.1 16.0 18.6 17.1

1982 20.5 16.3 19.0 17.4

1983 20.9 16.5 19.6 17.8

1984 20.5 16.1 19.3 17.4

1985 20.6 16.2 19.4 17.5

1986 21.4 16.9 20.2 18.3

1987 21.6 16.9 20.7 18.4

1988 21.2 16.5 20.4 18.1

1989 20.9 16.3 20.1 17.8

1990 20.7 16.1 20.2 17.7

1991 21.3 16.4 20.7 18.1

1992 20.8 16.1 20.4 17.8

1993 21.0 16.1 20.7 17.9

1994 20.8 16.0 20.4 17.7

1995 20.5 15.8 20.2 17.5

1996 20.8 16.0 20.7 17.8

1997 20.6 15.8 20.4 17.6

1998 20.9 16.0 20.8 17.8

1999 20.5 15.7 20.3 17.5

2000 20.5 15.7 20.3 17.5

2001 20.3 15.6 20.0 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 Passenger Light-Duty Trucks

Year Cars 1 and 2 3 and 4

1974 and

Earlier

0.67% 11.70% 0.01%

1975 1.55% 10.38% 0.00%

1976 1.37% 1.87% 0.00%

1977 0.93% 0.00% 0.00%

1978 1.14% 2.59% 0.01%

1979 2.69% 3.16% 0.11%

1980 3.90% 4.39% 0.06%

1981 7.06% 6.16% 0.13%

1982 5.10% 6.56% 2.56%

1983 2.41% 2.23% 2.09%

1984 1.62% 1.20% 1.69%

1985 0.97% 0.48% 1.35%

1986 0.32% 0.33% 1.24%

1987 0.27% 0.07% 0.82%

1988 0.01% 0.00% 0.72%

1989 0.04% 0.00% 0.83%

1990 0.04% 0.00% 0.96%

1991 0.13% 0.00% 1.29%

1992 0.06% 0.00% 1.15%

1993 0.03% 0.00% 1.45%

1994 0.01% 0.00% 1.11%

1995 0.06% 0.00% 1.15%

1996 and

Later

0.09% 0.00% 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 <=== Passenger Cars ===> <=== Light Trucks ===>

Year Gasoline Diesel Combined Gasoline Diesel Combined

1978 19.8 29.4 19.9 15.2 21.2 15.2

1979 20.1 27.1 20.3 14.7 21.1 14.7

1980 23.3 30.0 23.5 18.4 24.3 18.6

1981 24.9 29.9 25.1 19.7 32.0 20.1

1982 25.9 30.6 26.0 20.0 27.0 20.5

1983 25.8 30.8 25.9 20.7 27.0 20.9

1984 26.2 36.3 26.3 20.4 27.4 20.5

1985 26.9 34.2 27.0 20.5 26.1 20.6

1986 27.9 40.5 27.9 21.4 26.7 21.4

1987 28.0 30.5 28.1 21.6 25.6 21.6

1988 28.6 37.4 28.6 21.1 22.2 21.2

1989 28.1 44.3 28.1 20.9 22.6 20.9
-26- DRAFT

Appendix F

Merging Appendices B, C, and E

(ALL Fuel Economies Are Adjusted "Lab" 55/45)

Model <=== Passenger Cars ===> <=== Light Trucks ===>

Year Gasoline Diesel Combined Gasoline Diesel Combined

1975 --- --- 13.5 --- --- 11.6

1976 --- --- 14.9 --- --- 12.2

1977 --- --- 15.6 --- --- 13.3

1978 16.8 25.0 16.9 12.9 18.0 12.9

1979 17.1 23.0 17.2 12.5 17.9 12.5

1980 19.8 25.5 20.0 15.7 20.7 15.8

1981 21.2 25.4 21.4 16.7 27.2 17.1

1982 22.0 26.0 22.2 17.0 23.0 17.4

1983 21.9 26.2 22.1 17.6 23.0 17.8

1984 22.2 30.9 22.4 17.3 23.3 17.4

1985 22.9 29.1 23.0 17.4 22.2 17.5

1986 23.7 34.4 23.8 18.2 22.7 18.3

1987 23.8 25.9 24.0 18.4 21.8 18.4

1988 24.3 31.8 24.4 17.9 18.9 18.1

1989 23.9 37.7 24.0 17.8 19.2 17.8

1990 --- --- 23.7 --- --- 17.7

1991 --- --- 23.9 --- --- 18.1

1992 --- --- 23.6 --- --- 17.8

1993 --- --- 24.1 --- --- 17.9

1994 --- --- 24.0 --- --- 17.7

1995 --- --- 24.2 --- --- 17.5

1996 --- --- 24.2 --- --- 17.8

1997 --- --- 24.3 --- --- 17.6

1998 --- --- 24.4 --- --- 17.8

1999 --- --- 24.1 --- --- 17.5

2000 --- --- 24.2 --- --- 17.5

2001 --- --- 24.2 --- --- 17.3
-27- DRAFT

Appendix G

Estimating Missing Values for Appendix F

Model <=== Passenger Cars ===> <=== Light Trucks ===>

Year Gasoline Diesel Combined Gasoline Diesel Combined

1975 13.4 24.0 13.5 11.2 18.0 11.6

1976 14.8 24.0 14.9 12.1 18.0 12.2

1977 15.5 24.0 15.6 13.2 18.0 13.3

1978 16.8 25.0 16.9 12.9 18.0 12.9

1979 17.1 23.0 17.2 12.5 17.9 12.5

1980 19.8 25.5 20.0 15.7 20.7 15.8

1981 21.2 25.4 21.4 16.7 27.2 17.1

1982 22.0 26.0 22.2 17.0 23.0 17.4

1983 21.9 26.2 22.1 17.6 23.0 17.8

1984 22.2 30.9 22.4 17.3 23.3 17.4

1985 22.9 29.1 23.0 17.4 22.2 17.5

1986 23.7 34.4 23.8 18.2 22.7 18.3

1987 23.8 25.9 24.0 18.4 21.8 18.4

1988 24.3 31.8 24.4 17.9 18.9 18.1

1989 23.9 37.7 24.0 17.8 19.2 17.8

1990 23.6 32.4 23.7 17.6 20.6 17.7

1991 23.8 32.4 23.9 18.0 20.6 18.1

1992 23.5 32.4 23.6 17.7 20.6 17.8

1993 24.0 32.4 24.1 17.8 20.6 17.9

1994 23.9 32.4 24.0 17.6 20.6 17.7

1995 24.1 32.4 24.2 17.4 20.6 17.5

1996 24.1 32.4 24.2 17.7 20.6 17.8

1997 24.2 32.4 24.3 17.5 20.6 17.6

1998 24.3 32.4 24.4 17.7 20.6 17.8

1999 24.0 32.4 24.1 17.4 20.6 17.5

2000 24.1 32.4 24.2 17.4 20.6 17.5

2001 24.1 32.4 24.2 17.2 20.6 17.3
-28- DRAFT

Appendix H

Expanding Fuel Economy Estimates

From Appendix G to ALL Light-Duty Classes

Model <=== Gasoline-Fueled ===> <=== Diesel-Fueled ===>

Year LDGV LDGT12 LDGT34 LDDV LDDT12 LDDT34

1975 13.4 12.0 9.3 24.0 19.4 14.9

1976 14.8 13.0 10.0 24.0 19.4 14.9

1977 15.5 14.2 10.9 24.0 19.4 14.9

1978 16.8 13.9 10.7 25.0 19.4 14.9

1979 17.1 13.4 10.3 23.0 19.3 14.8

1980 19.8 16.8 12.9 25.5 22.2 17.1

1981 21.2 18.0 13.8 25.4 29.2 22.5

1982 22.0 18.3 14.1 26.0 24.7 19.0

1983 21.9 18.9 14.5 26.2 24.7 19.0

1984 22.2 18.6 14.3 30.9 25.0 19.3

1985 22.9 18.7 14.4 29.1 23.8 18.3

1986 23.7 19.6 15.0 34.4 24.4 18.8

1987 23.8 19.7 15.2 25.9 23.4 18.0

1988 24.3 19.3 14.8 31.8 20.3 15.6

1989 23.9 19.1 14.7 37.7 20.7 15.9

1990 23.6 18.9 14.6 34.2 22.1 17.0

1991 23.8 19.4 14.9 34.2 22.1 17.0

1992 23.5 19.0 14.6 34.2 22.1 17.0

1993 24.0 19.1 14.7 34.2 22.1 17.0

1994 23.9 18.9 14.6 34.2 22.1 17.0

1995 24.1 18.7 14.4 34.2 22.1 17.0

1996 24.1 19.0 14.6 34.2 22.1 17.0

1997 24.2 18.8 14.5 34.2 22.1 17.0

1998 24.3 19.0 14.6 34.2 22.1 17.0

1999 24.0 18.7 14.4 34.2 22.1 17.0

2000 24.1 18.7 14.4 34.2 22.1 17.0

2001 24.1 18.5 14.2 34.2 22.1 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 Weight Classes:

Year 2b 3 4 5 6 7 8A 8B

83 9.2 7.1 6.5 6.7 6.3 7.6 4.5 --

84 8.6 8.4 6.3 8.2 7.2 6.0 8.5 --

85 9.0 8.3 8.1 7.0 7.3 6.7 5.8 --

86 8.7 8.2 7.7 7.3 7.5 6.6 6.4 --

87 9.4 9.1 7.9 8.4 6.8 6.1 8.5 --

88 9.7 8.8 8.1 7.2 7.9 7.7 5.6 --

89 9.4 10.2 9.0 6.3 6.9 5.7 5.1 --

90 9.7 9.6 9.9 8.5 7.7 7.7 7.2 --

91 8.7 8.9 9.5 7.6 8.8 7.7 7.8 --

92 10.4 10.7 8.9 7.3 7.6 13.8 6.3 --

Summarized Diesel Heavy-Duty Vehicle Fuel Economies (mpg)

Model Weight Classes:

Year 2b 3 4 5 6 7 8A 8B

83 10.8 9.8 8.1 7.6 8.0 5.9 5.5 5.2

84 13.2 9.6 6.4 8.1 8.0 7.0 5.7 5.3

85 11.3 11.0 8.3 9.9 7.6 7.7 5.9 5.3

86 12.2 11.9 9.0 8.4 8.1 7.3 6.0 5.5

87 11.7 10.6 9.5 9.2 8.3 7.6 6.1 5.5

88 11.8 11.8 9.7 9.9 8.9 8.0 6.1 5.5

89 12.4 10.6 10.4 10.7 8.8 7.2 5.9 5.7

90 11.6 11.6 10.1 9.2 8.1 7.4 6.1 5.8

91 12.0 11.2 12.9 9.4 8.2 7.3 6.4 5.9

92 12.4 11.1 9.9 9.7 8.3 7.5 6.2 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,
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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 realworld 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 gasolinefueled 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).

GHG MOBILE6.3 Estimates EIA Estimates

Gasoline Diesel Gasoline Diesel Gasoline Diesel

1,048 247 991 274 1,124 310

1,295 1,265 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 CO2 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 lbs 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

lbs for Ranger and S-10 in 1991; and 1/2 ton full size pickups

are just under and over 6000 lbs. 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.

p14 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, pp1067-1077,

1991.

On the other hand, I once looked at CO2 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 lightduty" 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.