APPENDIX B: ESTIMATION METHODOLOGIES 
APPENDIX B 

APPENDIX B 

ESTIMATION METHODOLOGIES

INTRODUCTION 

The National Household Travel Survey (NHTS) is the nation’s inventory of local and long 

distance travel, according to the U.S. Department of Transportation. Between April 2001 and

May 2002, roughly 26 thousand households41 were interviewed about their travel, based on the

use of over 53 thousand vehicles. Using confidential data collected during those interviews,

coupled with EIA’s retail fuel prices, external data sources of test42 fuel economy, and internal 

procedures for modifying test fuel economy to on-road, in-use fuel economy, EIA has extended 

this inventory to include the energy used for travel, thereby continuing a data series that was

discontinued by EIA in 1994. This appendix presents the methods used for each eligible sampled

vehicle to 

1. provide three fundamental inputs crucial to developing annual household vehicles 

energy consumption and expenditures information: composite fuel economy, retail 

fuel price, and in-possession vehicle-miles traveled; 

2. adjust imputed composite fuel economy to calculate an on-road fuel economy;

3. adjust on-road fuel economy to calculate an in-use fuel economy based on actual

household driving characteristics; and, 

4. derive annual energy consumption and motor fuel expenditures information from 

these adjusted inputs. 

Such methods allow EIA to calculate estimates on the amount of and expenditures for energy

consumed by the nation’s vehicles operated for residential transportation. These estimates also 

include number and types of vehicles per household, and for each vehicle: annual miles traveled,

gallons of fuel consumed, type of fuel used, price paid for fuel, and fuel economy (gasoline

mileage). 

DISCLAIMER 

 41 The NHTS collected travel data from the civilian, non-institutionalized population of the United States. People living 

in medical institutions, prisons and in barracks on military bases were excluded from the sample. However, telephone 

numbers in dormitory rooms, fraternity and sorority houses were included so long as no more than 10 people shared the

same telephone number. 

42 Federal law requires automobile manufacturers to determine the fuel economy of new vehicles offered for sale in the 

U.S. This information is provided on a fuel economy label affixed to each vehicle’s window to help consumers make 

informed decisions regarding fuel economy when purchasing a new vehicle. While these labels may vary somewhat in

appearance, they must all provide the same information.

ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA &

TRENDS

126
APPENDIX B 

Had these results come from information 

supplied by respondents to the NHTS directly, 

there would be no reason to provide an explicit 

warning to users about the uncertainty of these 

data. Since NHTS did not collect information

on fuel economy, retail fuel price, or fuel type, 

EIA cautions readers that, for every single one

of the over 53 thousand sampled vehicles, all

energy and energy-related statistics in this 

report and associated tables, public-use files,

and analyses are constructed from imputed

information. Only light-duty passenger

vehicles are included in this report because

EIA has excluded motorcycles, mopeds, large

trucks, and buses in an effort to continue its 

past residential transportation series, which

was discontinued in 1994. 

In-Scope Households and Vehicles 

Not all NHTS sampled vehicles are

considered in-scope for this report. For this 

study, except where noted, we include only 

those vehicles that (1) fit the definition of 

light-duty residential passenger vehicles

and (2) belong in the sample of households 

having the “100-percent-household” 

national weight classification, as defined

by the User’s Guide of the NHTS. 

In 2001, the NHTS interviewed the 

members of 26,308 households that had 

53,275 vehicles at some point during the 

survey period. Of that vehicle total, about

80 percent, or 42,736 vehicles, conform to

EIA’s definition of a vehicle and also are

identified as belonging to a “100-percent 

household” by the NHTS, which is to say

that these households form a national

representative sample of respondents 

whose entire adult members were directly 

interviewed. Likewise, the sample number 

of households is reduced to 21,178 from

26,308, after excluding those households 

where one or more adult members chose 

not to respond. 

The calculation of energy-related statistics

– vehicle fuel consumption and expenditures – 

in this report occurred in several steps.

Multiple steps were required because 

respondents, when completing their NHTS 

survey questionnaires, were not directly asked

to report information necessary to derive their 

vehicle’s on-road, in-use fuel economy, nor 

were they asked to provide the type or price of 

the fuel that was used to power their vehicle(s).

Without all of these critical components, there 

is no way to determine a vehicle’s 

consumption of and expenditures for transportation fuel. With the use of confidential NHTS data

and other external data sources, EIA’s imputation procedures modeled these measures for most

sampled vehicles (see “In-Scope Households and Vehicles” text boxes for details). 

DATA SOURCES 

To derive vehicle-miles traveled (VMT); assign and adjust vehicle fuel economy (given in 

terms of miles per gasoline equivalent gallon (MPG)); compute vehicle fuel consumption, and 

assign fuel prices to calculate vehicle fuel expenditures, EIA relied on data from several federal 

agencies. These statistical procedures relied on confidential data from the U.S. Federal Highway 

Administration’s (FHWA) 2001 National Household Travel Survey (NHTS); the EIA’s 1985,

1988, and 1991 Residential Transportation Energy Consumption Survey (RTECS)43; the U.S. 

 43 This series was discontinued after EIA conducted the 1994 Residential Transportation Energy Consumption survey. 

ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS 127
APPENDIX B 

Environmental Protection Agency’s (EPA) fuel 

economy test results44; and the EIA’s retail

pump price series45 for 2001 and 2002. 

In-Scope Households and Vehicles 

(Continued) 

The NHTS has recommended that

some data applications, such as planning 

models, use only the national sample of 

“100-percent households.” EIA has kept 

with that recommendation for this report. 

For the definition of a light-duty

residential passenger vehicle used in this

report, see vehicle in the “Glossary” of this

report. 

PROCEDURES AND DEFINITIONS 

EIA’s purpose in partnering with the U.S. 

Department of Transportation was to enhance 

the use and usefulness of the January 2004 

release of the 2001 NHTS public-use file, 

augmenting it with energy-related data. Figure 

B1 depicts the estimation of those energyrelated statistics: VMT, vehicle fuel economy,

vehicle fuel consumption, and vehicle fuel 

expenditures. These steps were initially applied to each vehicle reported by households in the 

national sample of the NHTS. However, item nonresponse (mostly of crucial vehicle 

characteristics), incomplete fuel economy and sales data (generally for those vehicles having a 

gross vehicle weight rating heavier than 8,500 lbs), and the goal to update national estimates that 

conceptually compare to those found in EIA’s previous residential transportation studies – 1985, 

1988, 1991, and 1994 RTECS – guided the scope of EIA’s augmented vehicle data. The effect of

those inter-dependent challenges resulted in methodologies that applied only to light-duty 

passenger vehicles in households that are nationally weighted as “100-percent-household” by the

NHTS. 

First, the annual VMT was derived from the vehicle’s two odometer readings or imputed

using modeled data (see the NHTS User’s Guide 2-B.5. ODOMETER READING and 

APPENDIX J). Moreover, because vehicles are acquired and disposed of by sample households 

during the survey year, the annual VMT were subsequently adjusted to reflect the period of the

survey year in which the household “owned or used” the vehicle.46 Second, the annual on-road

 44 Fuel economy test values and vehicle production sales data were received from the U.S. Department of

Transportation, National Highway and Traffic Safety Administration for model year’s 1978 through 2002. 

45 Energy Information Administration. Forms EIA-782A, "Refiners'/Gas Plant Operators' Monthly Petroleum Product 

Sales report," and EIA-782B, "Resellers'/Retailers' Monthly Petroleum Product Sales Report." Form EIA-888, “OnHighway Diesel Fuel Price Survey.” Form EIA-895, "Monthly Quantity and Value of Natural Gas Report." Form

EIA-826, "Monthly Electric Utility Sales and Revenue Report with State Distributions." 

46 Thus, “number of vehicles” and “per vehicle” statistics in this report are displayed in units of vehicle-years. For 

example, if a vehicle was sold half-way through the survey year (i.e., household “owned” vehicle for six months of the 

year), then this vehicle is counted as 0.5 vehicle-years, while a vehicle “owned” for 365 days would equate to 1 

vehicle-year. Likewise, a vehicle purchased during the survey period would also be prorated to its time of household 

use, as measured from its acquisition date and the end of the survey year. These dates are available from odometer 

reading flags. Hence, vehicle statistics in this report will differ from those published directly from NHTS public-use 

data because the duration of a vehicle’s possession by a household is taken into account, calculating an accurate level

of energy and vehicle use. Since NHTS did not collect an end-of-year vehicle inventory, a disposition date for each

vehicle is not known. 

128 ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS
APPENDIX B 

fuel economy, reported in terms of MPG, was estimated using questionnaire responses47, EPA

fuel economy test results, and the period between May 1, 2001 and April 30, 2002 that the vehicle 

was in use.48 The MPG values were adjusted to account for the difference between EPA test 

values and on-road, in-use values. Third, estimated vehicle fuel consumption was derived by

dividing the prorated VMT by the estimated MPG. Then, multiplying the vehicle’s fuel 

consumption by its fuel price, on a monthly basis, derives motor fuel expenditures.

Unfortunately, the NHTS did not collect the vehicle’s motor fuel prices via fuel purchase diaries. 

Instead, each NHTS vehicle was assigned a retail price based on its imputed fuel type. All price

information, with the notable exception of fuel tax rates for gasoline obtained from FHWA’s 

Highway Statistic reports, was obtained from the EIA’s transportation fuel price series. 

The following sections of this appendix describe the estimation procedures used for

calculating a vehicle’s monthly VMT, MPG, fuel consumption, fuel price, and fuel expenditure. 

The following terms are used throughout this report: 

Fuel Economy Term Definition 

EPA Composite MPG The EPA dynamometer test procedure, performed on pre-production 

prototype vehicles, yields separate test values for EPA city and highway

MPG. These city and highway MPG are often combined to form the

"composite" MPG. 

On-Road MPG A Composite MPG that was adjusted to account for the shortfall 

between the test value and the fuel economy actually obtained on the

road. The adjustment did not take into account the driving patterns of 

individual drivers and seasonal differences. 

In-Use MPG MPG that were adjusted for seasonal differences and annual miles 

driven. Vehicles that are driven relatively few miles during the year are

assumed to be driven mostly on short trips that involve frequent stops. 

Vehicles that are driven relatively many miles are assumed to be driven

mostly on long trips where few stops are needed. 

MPG Shortfall A measure of the difference between actual on-road MPG and the EPA

laboratory test MPG, expressed as the ratio of test MPG to on-road 

MPG. 

 47 FHWA/BTS collected make (MAKECODE), model (MODLCODE), model year (VEHYEAR), and 8 categories of

vehicle type (VEHTYPE), as given in Section B: Vehicle Data of the 2001 NHTS questionnaire. The collection of 

Vehicle Identification Numbers (VIN) would have provided a more accurate and richer source of vehicle

characteristics. It is not known whether VINs will be collected in future survey cycles of the NHTS. 

48 For 2002 model year vehicles, the NHTS calculates odometer-based VMT (BESTMILE) for the entire 12-month 

time period. EIA, however, adjusts all in scope, odometer-based VMT estimates to represent the time period in which

the vehicle was “owned” by the household; this is an adjustment that serves to equate both travel and energy use with 

the vehicle’s availability.

ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS 129
APPENDIX B 

EPA test value data from NHTSA are restricted to vehicles that are used to derive Corporate

Average Fuel Economy under Title V of the Motor Vehicle Information and Cost Savings Act 

(15 U.S.C. 1901, et seq.) with subsequent amendments and Subtitle VI (49 U.S.C. 329). 

Corporate Average Fuel Economy (CAFE) is the sales-weighted average fuel economy,

expressed in miles per gallon, of a manufacturer’s fleet of passenger cars or light trucks with a

gross vehicle weight rating (GVWR) of 8,500 lbs. or less, manufactured for sale in the United

States, for any given model year.49 Fuel economy is defined as the average mileage traveled by a

vehicle per gallon of gasoline (or equivalent amount of other fuel) consumed as measured in

accordance with the testing and evaluation protocol set forth by Environmental Protection 

Agency (EPA). 

Manufacturers also perform their own fuel economy tests of new vehicle models and submit 

the results to EPA. EPA is responsible for conducting its own tests or verifying the

manufacturers’ dynamometer tests. EPA also is responsible for compiling the production data 

from manufacturers’ reports and furnishing CAFE results to NHTSA. 

Fuel economy test data from the manufacturers and EPA serves as the starting point for both 

CAFE values and real-world fuel economy projections. For CAFE, the test data are adjusted 

upward to account for any credits for dual-fuel alternative fuel vehicles (AFV) and dedicated 

AFV, and for passenger cars only, is also adjusted upward for credits available to manufacturers

to account for test procedure changes since the CAFE program was established. For NHTS and

this report, such credits and their associated upward adjustments were removed, if indicated by

NHTSA. 

Table B1. Sample Counts of Residential Passenger Vehicles by Type and Model Year, 2001

NHTS Model Year

Vehicle Type

Vehicle 

Type 

Code Pre-1978

1978 - 

2001 2002

Not 

Ascertained Total

Automobile 01 842 22,468 381 571 24,262

Van 02 41 3,885 72 108 4,106

Sport Utility Vehicle 03 73 4,958 221 120 5,372

Pickup Truck 04 510 7,692 140 259 8,601

Recreation Vehicle 06 49 330 2 14 395

Total 1,515 39,333 816 1,072 42,736

Source: U.S. Department of Transportation, Federal Highway Administration, 2001 National Household Travel Survey, 

January 2004 release. (Washington, DC). Note: Not all NHTS sampled vehicles are displayed -- only light-duty 

residential passenger vehicles are tabulated here, fitting this report’s definition of a vehicle (see “Glossary” for details). 

Since the NHTS is a national survey, it collected data from a nationally representative sample

of households to derive statistically reliable travel estimates at the national, region (4) and 

division (9) levels. Sample data in the NHTS are generally not adequate to provide state or 

smaller area-specific estimates. However, the 2001 NHTS sample included several jurisdictions

 49 These vehicles are conceptually consistent with 2001 NHTS sample vehicles having a vehicle type of “01”

(Automobile), “02” (Van), “03” (Sport Utility Vehicle), “04” (Pickup Truck). EPA does not provide test data for

vehicles such as the Ford Excursion, Hummer H1 and Hummer H2 because they have a GVWR greater than 8,500 lbs. 

130 ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS
APPENDIX B 

where additional sample households were purchased and interviewed. The jurisdictions that

purchased these additional samples are referred to as the “add-on” areas. There are nine add-on 

areas: Baltimore Metropolitan Planning Organization (MPO), Maryland; Des Moines MPO, 

Iowa; Edmonson, Carter, Pulaski, and Scott Counties, Kentucky; Lancaster MPO, Pennsylvania; 

Oahu MPO, Hawaii; State of Hawaii, except Oahu; State of New York; State of Texas; and State 

of Wisconsin.50 These additional sample cases were not included in developing the energy 

consumption and expenditures data, nor were any data developed for or applied to these vehicles. 

Figure B1. Estimation Schematic

From NHTS/EIA:

Vehicle-Miles

Traveled (VMT)

From EPA/NHTSA:

Fuel Economy in miles per 

equivalent gallon (MPG) 

From EIA:

Vehicle Fuel Price in current 

dollar per equivalent gallon

(DPG)

Merge 

Vehicle Consumption (gal) Merge 

MPG

VMT During Possession =

Vehicle Fuel Expenditures

= DPG • Gallons

Adjust Fuel Economy to 

on-road, in-use values 

Note: NHTS – National Household Travel Survey, EPA – Environmental Protection Agency, EIA – Energy

Information Administration, and NHTSA – National Highway Transportation Safety Administration.

VEHICLE MILES TRAVELED 

When possible, VMT of sample vehicles were determined by taking the difference between

two odometer readings, which spanned a period of time, and expanding this difference to cover

the year, or 365 days. The exact methods for deriving annual VMT are discussed in Appendix J in

the NHTS User’s Guide, as written by the Oak Ridge National Laboratory (ORNL), Engineering 

Science Technology Division, Center for Transportation Analysis. 

For 1,262 vehicles, the work conducted by ORNL did not result in a viable annual VMT 

estimate. The lack of such a VMT value is indicated in the NHTS public-use file with an

insertion of negative value codes for these vehicles’ value of “BESTMILE,” which is the public-

 50 See http://www.bts.gov/external_links/government/metropolitan_planning_organizations.html for a complete list. 

ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS 131
APPENDIX B 

use variable denoting ORNL’s annual VMT estimate. According to the NHTS public-use data, 

roughly 3 percent of the 42,736 vehicles deemed in-scope lack annual VMT estimates, all of 

which are coded as “8” or “9” in the January 2004 NHTS public-use data by the “BEST_FLG”

public-use variable. To derive annual VMT estimates for these 1,262 vehicles, two methods were 

employed by EIA: (1) a standard annualization procedure to inflate spanned odometer readings

(used for 626 vehicles) and simple multiple regression models (used for 636 vehicles). 

For 626 vehicles, the NHTS public-use data file contains both valid odometer readings and

dates (month, day and year) and, more importantly, these vehicles’ reading dates spanned a

period greater than 60 days. Thus, VMT for nearly half of the 1,262 vehicles were derived from 

EIA’s standard annualization procedure. This procedure has been employed by previous EIA 

studies: RTECS in the years 1985, 1988, 1991, and 1994. Specifically, the annual VMT for 

vehicle i was computed by dividing the reported span of odometer readings with the monthly

VMT fractions, Fj, of the time period covered by the ith vehicle’s travel, as determined from the

standard distribution given in Table B2 of this appendix. Because odometer readings were 

recorded with the exact day of the month, some monthly VMT fractions were prorated to

correspond to the exact span of odometer readings.51 Generally, this resulted in prorated VMT

fractions values of both starting and ending monthly fractions. Moreover, for vehicles reporting a 

span of odometer readings greater than 365 days, annual VMT were reduced by these same 

monthly driving fractions, which were prorated, as needed. It is important to point out that exact 

odometer reading dates were not released in the NHTS public-use files; rather, EIA accessed

confidential NHTS data for assigning odometer reading dates. 

For 636 vehicles, no odometer information was available. For most of these vehicles, annual

VMT were imputed using multiple linear regressions in which the independent variable was the 

respondent’s self-reported estimate of annual VMT obtained during the NHTS interview. If, on 

the other hand, the NHTS did not collect the vehicle’s self-reported annual VMT, then EIA used a 

selection of predictors drawn from the household and vehicle’s characteristics, which include the 

number of drivers, ratio of the number of vehicles per driver, household income, age of vehicle, 

type of vehicle, and household composition. 

IN-POSSESSION VEHICLE-MILES TRAVELED 

EIA also extended the logic of computing a vehicle’s VMT by estimating the period of time 

that the vehicle was in the household’s possession. This was done in multiple steps using publicuse data provided by FHWA. Once annual VMT were obtained either through the work 

completed by ORNL or the two approaches (i.e., standard annualization or multiple regressions) 

undertaken by EIA, each vehicle’s annual VMT value was adjusted to correspond to the time

period that the vehicle was in the possession of the sample household during the survey year, 

which started on May 1, 2001 and ended on April 30,2002. Using a vehicle’s acquisition and

disposition dates, as derived from NHTS interview contact and odometer reading dates or other 

relevant contact information on survey follow-up procedures52, an in-possession VMT value was

 51 Based on an EIA review of the NHTS public-use file that was released in January 2004, only month and year dates 

are available for odometer readings. However, EIA was provided access to actual odometer reading dates, including

month, day and year of these readings. 

52 Follow-up contacts with NHTS respondents were undertaken within a set procedure, according to correspondences

with NHTS contractor, Mark Freedman of Westat. 

132 ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS
APPENDIX B 

calculated based on standard monthly driving fractions, Fj.

53 By simply multiplying the annual

VMT by the sum of the monthly driving fraction, prorated as needed, a VMT during possession

was computed. Only 38,639 vehicles, or 90 percent of the eligible 42,736 vehicles, were retained

for the entire survey period (365 days). As discussed later in this appendix, these VMT-duringpossession values form the basis of both energy consumption and expenditures for residential 

passenger vehicle use. 

VEHICLE FUEL ECONOMY 

Fuel economy (MPG) must be estimated for each NHTS sample vehicle in order to estimate 

each vehicle's fuel consumption for the survey year. Then, fuel consumption is estimated by 

dividing the VMT for time of possession54 by the MPG.55 The NHTS neither obtained actual fuel

consumption data nor on-road MPG from fuel purchase diaries maintained by the respondents.

Because NHTS did not require these data or diaries, MPG values were estimated using EPA

laboratory test MPG that were adjusted to account for differences between actual on-road MPG 

and the EPA test MPG. This difference is known as MPG "shortfall." Lax, 198756; Mintz, 199357; 

and Reichert, 200058, investigated the feasibility of using shortfall-adjusted MPG in a household 

survey. The Lax study verified that the method yielded unbiased MPG, when using a database 

from a 1984 fuel purchase diary study performed by NPD Research, Inc. The adequacy of current

shortfall adjustment methods is sufficient for late 1980 through early 1993's motor vehicle model 

years also (RTECS Technical Note 5).59 For the 2001 NHTS, the adequacy of shortfall 

adjustments has been presumed for 1994 through 2002’s motor vehicle model years. 

 53 To ensure that the distribution of average monthly vehicle miles traveled given in Table B2 reflected 2001 driving

patterns, EIA compared those fractions with the 2001 FHWA’s highway-based values. No significant differences were 

found; however, the events occurring in September 2001 and soon thereafter may have unknown contributions to travel 

behavior patterns not shown here. 

54 While the NHTS public-use data, which was released on January 2004, assumes a fixed 12-month period (starting on

May 1, 2001 and ending April 30, 2002) EIA has modified these same data in an attempt to compute the fraction of the 

year in which the household had “actual” possession of the vehicle. Because NHTS did not conduct an end-of-year

audit of a household’s vehicle stock, not all changes in vehicle stock are known.

55 The 2001 NHTS was conducted over the 14-month period from March 2001 to May 2002. Unfortunately, that timing

turned out to be problematic due to the September 11, 2001 terrorist attacks on the World Trade Center in New York 

and the Pentagon in Washington, DC. These attacks disrupted transport services for months, especially curtailing longdistance travel. It is not certain what impacts the attacks had on urban travel, but it seems likely that both the amount of

travel and modal choice were affected. That may have distorted the survey results to some unknown extent. 

Information courtesy of John Pucher and John L. Renne, Transportation Quarterly, Vol. 57, No. 3, 2003. 

56 Lax, D. 1987. “Feasibility of Estimating In-Use Vehicle Fuel Efficiency from Household Survey Data.” Research 

performed under contract for ORNL/DOE/EIA. Energy and Environmental Analysis Inc., Arlington, VA. 

57 Mintz, M., A. Vyas, and L. Conley, 1993. “Differences Between EPA-Test and In-Use Fuel Economy: Are the 

Correction Factors Correct?” Transportation Research Record 1416, pp. 124-130, Transportation Research Board, 

National Research Council, Washington, DC. 

58 Reichert, J. 2000. “Change in Method for Estimating Fuel Economy for the Residential Transportation Energy

Consumption Survey,” Energy Information Administration on www.eia.doe.gov/emeu/rtecs/contents.html. 

59 Harrison, I.M. “VMT 1991 Patterns,” Residential Transportation Energy Consumption Survey Technical Note 5, 

unpublished document. (Washington, DC).

ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS 133
APPENDIX B 

The NHTS sample vehicles were assigned EPA test MPG from the NHTSA Corporate

Average Fuel Economy files. Each record of the NHTSA files contained an EPA Composite 

MPG (i.e., an unadjusted 45 percent highway and 55 percent city weighted estimate) for each 

unique combination of vehicle attributes within a given manufacture, model/carline, type and 

model year. These attributes included (1) number of cylinders, (2) cubic inches of engine

displacement (CID), (3) type of transmission (manual or automatic), and (4) fuel metering 

(gasoline, diesel, electric, natural gas, duel-, or flexible-fuel vehicle).60 Each record of the

NHTSA files also contained the number of vehicles sold, in thousands of vehicles, for each

unique combination of attributes. The vehicle attributes available to assign a Composite MPG for

sample vehicles were the ones collected for each NHTS vehicle. Specifically, NHTS queried 

respondents on their vehicle’s make, model, vehicle type, and model year attributes. Hence,

merging, assigning and statistical linking to NHTSA’s Corporate Average Fuel Economy files 

were restricted to those four attributes. If, in the future, NHTS were to collect Vehicle

Identification Numbers (VIN), then these linking procedures might be performed on a more 

robust set of vehicle attributes.61

NHTSA files served multiple purposes. In addition to assigning a Composite MPG, the 

NHTSA files were used to impute “missing” vehicle attributes: fuel metering and engine type for

purposes of assigning an appropriate fuel price. Based on the limited set of vehicle attributes 

obtained from the NHTS questionnaire, several records from the NHTSA files were usually found

to be potential “matches” to a given sample vehicle. A matching record was chosen from among

the several applicable ones, with probability proportional to sales, using the sales figures on the 

NHTSA files. Once chosen, a record provided (1) EPA Composite MPG, (2) fuel metering, and

(3) engine type. Although more attributes were available for selection, EIA limited its matched 

attributes to those required to assign an appropriate fuel price to a sample vehicle. Of the 42,736 

eligible vehicles, EIA selected a matching record for 39,879 vehicle, or 93 percent. This 

matching routine commonly resulted in 1-to-many record linkages (see Figure C1 for more

details). 

For the remaining 7 percent of in-scope sample vehicles, EIA employed expert knowledge 

and hot decking of median Composite MPG values for assigning on-road, in-use MPG values to

510 vehicles and Composite MPG to 2,347, respectively. Hot-decking techniques were 

sequentially executed, based on the vehicle’s characteristics, such as make, model, model year, 

and vehicle type. 

The EPA Composite MPG are just the starting point for fuel economy computations. For the 

2001 NHTS, EIA employs a sequential adjustment procedure in which the EPA Composite MPG

are adjusted first to an on-road MPG, and then to an in-use MPG. 

 60 NHTSA file records do not include whether the vehicle's emissions control package met Federal or California 

standards.

61 VINs may be decoded to yield the vehicle attributes, by use of the Highway Loss Data Institute's "Vindicator" 

software. 

134 ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS
APPENDIX B 

THE EPA COMPOSITE MPG 

Beginning in the early 1970's, EPA measured fuel economy from tests that were conducted

on a dynamometer to simulate actual driving conditions. By 1975, EPA had incorporated separate 

“city” and “highway” driving cycles into the test. The city and highway MPG were combined to

form a “composite” MPG that was then weighted according to sales of the production vehicles in 

order to assess compliance with Corporate Average Fuel Economy (CAFE) standards. The EPA 

Composite MPG is based on the assumption of a "typical" vehicle-use pattern of 55 percent city 

driving and 45 percent highway driving, and has become a convenient single fuel economy

measure for analytical and regulatory purposes. 

The EPA Composite MPG62 is defined as: 

(EPA city) (EPA hwy)

(EPA 55/45)

MPG

1 0.45

MPG

1 0.55

1 MPG

• + •

= (1) 

where: 

MPG(EPA 55/45) denotes the composite MPG; MPG(EPA city) denotes the fuel economy when 

vehicle use pattern is city driving only; and, MPG(EPA hwy) denotes the fuel economy when vehicle 

use pattern is highway driving only. 

Because separate city and highway fuel economy estimates were not available on the NHTSA 

files, a single “shortfall” adjustment factor was derived, approximating the adjustments given in

the following sections. 

FUEL ECONOMY SHORTFALL 

Fuel economy shortfall occurs when the fuel economy that is actually obtained while using

the vehicle is lower than the EPA test results. Reasons for this shortfall are (1) a result of the 

differences between EPA test vehicles and the vehicles actually in use and (2) the differences

between EPA procedures for simulated driving conditions and actual driving conditions. For 

example, EPA test vehicles are prototypes that do not contain the wide variety of powerconsuming accessories often found on vehicles sold to consumers. The test procedures also do not 

simulate the actual driving conditions that affect fuel economy such as speed and acceleration of

individual drivers, road conditions, weather, and traffic. In the 2001 NHTS, adjustments for this 

fuel economy shortfall were made to the composite MPG (MPG(EPA 55/45)) that were assigned to 

the sample vehicles. 

Fuel economy shortfall was expressed in terms of the "Gallons per Mile Ratio" or GPMR: 

i

i (EPA55/45)

i

MPG

MPG GPMR = (2)

 62 Specifically, the following formulas, as stated in Part 600, Subpart F, §600.207-86, §600.208-77, §600.209-85, 

§600.510-86 of the 7-1-1994 edition of the 40 CFR, are identified for these calculations. 

ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS 135
APPENDIX B 

where: 

GPMRi denotes Gallons per Mile Ratio for the ith vehicle; MPGi denotes the on-road MPG or 

in-use MPG for the ith vehicle, depending on the analysis; and, MPGi (EPA 55/45) denotes the EPA 

Composite MPG applicable to the ith vehicle. 

Figure B2. Miles per Gasoline Equivalent Gallon Adjustment Procedures 

EPA Composite 

MPG

Adjusted for Fuel 

Efficiency Shortfall Using

Combined Discount Factor

Composite OnRoad MPG 

Adjusted for Individual

and Seasonal 

Circumstances 

Final In-Use

MPG

If GPMRi = 1 then there is no perceived shortfall. If GPMRi > 1 then there is a shortfall for 

vehicle i. That is, the on-road or in-use fuel economy is less than the fuel economy indicated by 

the EPA Composite MPG. Note that GPMRi can represent shortfall with respect to either the onroad or in-use MPGi, depending on the analysis being performed. GPMRi is commonly chosen as

a measure of shortfall as opposed to MPGi for the following reasons: 

• A shortfall adjustment is most often thought of as a correction factor, or 

multiplicative constant, rather than as an additive correction. GPMRi satisfies this 

convention.

• Shortfall is usually dependent on a vehicle's fuel economy level. That is, shortfall is 

usually higher at high levels of MPG(EPA 55/45) than at low levels of MPG(EPA 55/45). 

136 ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS
APPENDIX B 

Therefore, it is more informative to express the amount of shortfall relative to

MPG(EPA 55/45) rather than as an absolute quantity.

• GPMRi is a linear function of MPG(EPA 55/45) and can be modeled using ordinary least 

squares linear regression. 

• GPMRi is a transformation that stabilizes error variances for the purposes of least

squares linear regression. 

THE ON-ROAD MPG 

On-road MPG is a composite MPG that was adjusted to account for the shortfall between the 

EPA fuel economy and the actual fuel economy obtained on the road. 

The EPA developed two general procedures for adjusting MPG(EPA 55/45) to an on-road value. 

One procedure bases the size of the adjustment on specific technology features of the vehicle. The 

other procedure uses just two MPG discount factors, one to adjust the EPA highway estimate, the

other to adjust the city estimate. These two factors are used for all vehicles, regardless of

technology class. For our purposes, we approximated the earlier procedure with a single 

adjustment factor. 

Either of these procedures could have been approximated to adjust MPG(EPA 55/45) to an onroad MPG value for use in the 2001 NHTS. Since both procedures were unbiased for trucks, the 

choice as to which to employ in the 2001 NHTS should be based on their performance with cars.

According to the 1994 RTECS, the adjustment based on discount factors seemed to be less biased

than the Technology-Specific Adjustment. Further, the discount factors are also less expensive 

since they do not require collection or imputation of information on fuel delivery system and 

drive-train. Because of these reasons the Discount Factors Adjustment Method was selected for

approximation. 

SHORTFALL ADJUSTMENT BASED ON DISCOUNT FACTORS 

EPA's discount factors have widespread appeal because of their simplicity (Hellman and 

Murrell, 198563; Hellman and Murrell, 198464). The factors are 10 percent for city MPG and 22 

percent for highway MPG. That is, for any vehicle i, 

i(on - road, EPA hwy) i(EPA hwy)

i(on - road, EPA city) i(epa city)

MPG 0.78 MPG

MPG 0.90 MPG

= •

= • (3)

These discount factors are the ones used to produce the "sticker" MPG figures seen on 

vehicles on dealer lots, and are used to produce the DOE/EPA Gas Mileage Guide. The analysis

 63 Hellman, K.H., and Murrell, J.D. 1985. "On the Stability of the EPA MPG Adjustment Factors." Society of

Automotive Engineers Technical Paper Series, SAE Paper No. 851216, Warrendale, PA. 

64 Hellman, K.H., and Murrell, J.D. 1984. "Development of Adjustment Factors for the EPA City and Highway MPG 

Values." Society of Automotive Engineers Technical Paper Series, SAE Paper No. 840496, Warrendale, PA. 

ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS 137
APPENDIX B 

behind the development of these factors was performed on a conglomerate database with data

from Ford Motor Company, General Motors, Chrysler Corporation, DOE, and EPA. The database 

contained approximately 38,000 vehicle records with model years from 1979 through 1981 with 

some 1982 models included. The database contained predominately American-made vehicles, but

also included foreign vehicles as well. The technology mix was dominated by rear-wheel drive 

and carbureted vehicles, but contained some vehicles with front-wheel drive or fuel injection. 

Vehicle records contained make, model, year, vehicle characteristics, the MPG as measured on 

the road, MPG(EPA city), and MPG(EPA highway). The database also included the driver's perceptions of 

the proportion of their travel that was mostly urban (so called “city fraction”), and their average 

miles driven per day (AMPD). 

Fuel economy shortfall is affected by the vehicle use pattern: frequent starts and short trip 

lengths characterize city-driving pattern, while highway-driving pattern is characterized by

infrequent starts and long trips. AMPD is a good surrogate variable for representing these 

different driving patterns. 

The city-driving pattern was characterized by AMPD from 5 to 22 miles per day, while the 

highway-driving pattern was characterized by AMPD's from 15 to 105 miles per day (Hellman

and Murrell, 1984). City fraction and AMPD were used to split the data into two sets, one for 

development of the city discount factor, the other for development of the highway factor. The 

“city” and “highway” data sets were each stratified by vehicle technology classes. Linear

regression was performed within each stratum. GPMR was regressed on city fraction, AMPD,

MPG(EPA 55/45), odometer reading, and average temperature. The fitted models were then weighted 

and combined across vehicle technology strata, to produce a single "city" shortfall model and a 

single "highway" shortfall model. The weights were used to increase the influence of those

models that represented technology mixes expected to become more prominent in the future (e.g., 

front-wheel drive and fuel-injected vehicles). The discount factors were derived from the two

weighted models set at average or typical values of the independent variables. 

For each NHTS vehicle, if and only if separate city and highway MPG were available,

discounted city and highway on-road MPG may be computed and then combined to form an onroad 55/45 composite as follows: 

(on - road, EPA city) (on - road, EPA hwy)

(on - road,55/45)

MPG

1 0.45

MPG

1 0.55

1 MPG

• + •

= (4) 

Then, a shortfall ratio based on EPA discount factors would be computed for each NHTS

vehicle as follows: 

i(on - road,55/45)

i(EPA 55/45)

i(on - road)

MPG

MPG GPMR = (5)

Unfortunately, separate on-road city and highway test MPG were not available from the

NHTSA Corporate Average Fuel Economy files. Although a literature review reveals that 

shortfalls vary for particular vehicles or groups of vehicles, we have used a combined shortfall 

138 ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS
APPENDIX B 

estimate of 15 percent, equating to a GMPRi(on-road) of 1/0.85, which may also be written to reveal

that MPGi(on-road, 55/45) = 0.85 • MPGi(EPA55/45).

65

THE IN-USE MPG 

In-use MPG are MPG that are adjusted for individual driving circumstances. The on-road 

adjustments to MPG(EPA 55/45) discussed in the previous sections were “general” in that they did 

not take into account any effects on fuel economy that are due to the driver's individual

circumstances. They, instead, utilized general attributes such as the technology features of the 

vehicle and average driving conditions. Fuel economy shortfall estimates can be refined for an

individual vehicle by taking into account the following “in-use” effects. 

• Urban versus rural driving pattern. That is, frequent starts and short trips as opposed

to infrequent starts and longer trips. As mentioned in the previous section, a useful 

single variable for representing this effect is AMPD. High AMPD's usually represent 

mileage accumulated on the highway. 

• Traffic congestion, which increases with population density. 

• Seasonal temperature variations, especially for gasoline-carbureted vehicles. 

• Humidity, which together with temperature affects air-conditioner use. 

• Differences among geographic areas of the country. 

• Altitude. 

• Wind. 

• Road gradient and road surface conditions. 

Additionally, the seasonal change in gasoline composition and the mechanical condition of 

the sample vehicles affect on-road fuel economy. Both of these effects are unknown. More

importantly, EIA has made no attempt to account for these unknown effects. 

However, this appendix does address some of the individual vehicle influences. In general, 

the first four items are considered the most significant in-use influences (Crawford, 1983).66 In

the cited study, shortfall variations as high as 25 percent or more occurred over the range of 

typical AMPD. Shortfall was 16 percent higher in urban areas than in completely uncongested

areas, and was 12 percent higher in suburban areas. Shortfall varied seasonally (i.e., monthly) by

7 percent in the South and by 13 percent in the North.

 65 Hellman, K.H. and Murrell J.D., June 1982. “Why Vehicles Don't Achieve EPA MPG On the Road and How That 

Shortfall Can Be Accounted For,” Society of Automotive Engineers Technical Paper Series, SAE Paper 820791. 

66 Crawford, R. 1983. "Seasonal and Regional MPG as Influenced by Environmental Conditions and Travel Patterns." 

Research performed under contract for U.S. DOE. Energy and Environmental Analysis, Inc., Arlington, VA. 

ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS 139
APPENDIX B 

Regression models were developed (Crawford, 1983) for use in adjusting GPMRi(on-road) to an 

in-use shortfall employing measurements of several in-use effects as the independent variables. 

The regressions yielded a shortfall adjustment that was an additive one, which may be written

as follows: 

GPMRij(in - use) = GPMRi(on - road) + δij (6)

where GPMRij(in-use) denotes the in-use shortfall ratio estimate for the ith vehicle during the 

j

th month (j = 1, 2, ... 12); GPMRi(on-road) denotes the combined shortfall ratio fixed for the ith

vehicle; and, δij denotes the adjustment calculated for the ith vehicle during month j, from the a

regression model. 

One regression model from the Crawford reference that is appropriate for use in NHTS is as

follows: 

⎥

⎦

⎤ ⎢

⎣

⎡ ⎟

⎠

⎞ ⎜

⎝

⎛ ⎟

⎠

⎞ ⎜

⎝

⎛

⎥

⎦

⎤ ⎢

⎣

⎡ ⎟

⎠

⎞ ⎜

⎝

⎛ ⎟

⎠

⎞ ⎜

⎝

⎛

⎥

⎦

⎤ ⎢

⎣

⎡ ⎟

⎠

⎞ ⎜

⎝

⎛ ⎟

⎠

⎞ ⎜

⎝

⎛

• • + •

• • + • +

= • − +

6

jπ 0.031 cos

6

jπ SOUTH 0.030 sin

6

jπ 0.075 cos

6

jπ NORTH 0.050 sin

35.6

1

AMPD

1 δ 3.296

ij

ij

 (7) 

where AMPDij = Average Miles per Day for vehicle i and month j, typically 35.6 (i.e., 13,000 

miles per year); NORTH = 1 if the household is in the North, otherwise NORTH = 0 if the 

household is not in the North; and, SOUTH = 1 if the household is in the South, otherwise 

SOUTH = 0 if the household is not in the South. 

This regression model was chosen because the independent variables that are important in 

explaining shortfall were readily available from the 2001 NHTS data, using BESTMILE and the

distribution of average monthly vehicle miles travel fractions found in Table B2. The model had 

two components. One component involved AMPDij and represented the influence of individual 

driving patterns for a given vehicle and month. The other component represented the change in

shortfall that occurred throughout the seasons, due to the annual temperature cycle. The original 

regression equation also contained a minor term that accounted for the influence of airconditioner use during hot, humid weather. This term was dropped in the estimations because it 

involved the rather complex computation of “Discomfort Index” from NOAA weather records, 

and the slight additional precision was judged insufficient to warrant the additional processing 

expense. Additional terms representing geographic regional effects, and the natural logarithm of 

population density (people per square mile, to represent the influence of traffic congestion) were

not considered because of the computational cost. 

Once a GPMRij(in-use) was estimated it was used to estimate the final in-use fuel economy for

vehicle i and month j as follows: 

ij(in - use)

i(EPA 55/45)

ij(in - use)

GPMR

MPG MPG = (8)

140 ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS
APPENDIX B 

The regression equation had separate seasonal components for the “North” and “South” 

because the difference between the winter shortfall and the summer shortfall was greater in the

North than in the South. This difference can be seen in the model parameters. To define the North 

and South geographic areas the continental United States were divided into 97 two-digit ZIP 

Code regions. These regions were grouped to form two aggregate regions ("North" and "South") 

according to average winter and summer temperatures, and seasonal shortfall trends. 

ANNUAL VEHICLE FUEL CONSUMPTION

In the 2001 NHTS, annual consumption was calculated by dividing the annual VMT by the

annual MPG. The derivation of the “annualized” VMT is given in Appendix J of the NHTS 

User’s Guide. 

The MPGij(in-use) shown in the above section about fuel economy estimation procedures were 

final estimates of monthly in-use fuel economies for vehicle i, and could have been used for 

estimating monthly fuel consumptions and expenditures, if monthly VMT were known. 

Unfortunately, NHTS only collected data to annualize VMT. Nevertheless, the 2001 NHTS still 

made use of the MPGij(in-use) by disaggregating the annualized VMT of sample vehicles into 

monthly VMT, using monthly VMT driving fractions from the standard distribution in Table 

B2.67

Table B2. Distribution of Average Monthly Vehicle-Miles Traveled Fractions 

Monthj Average VMT per Vehicle Fj

January 688 0.0728

February 697 0.0738

March 771 0.0816

April 783 0.0829

May 832 0.0880

June 847 0.0896

July 868 0.0919

August 872 0.0923

September 800 0.0847

October 802 0.0849

November 756 0.0800

December 734 0.0777

Total 9,450 1.0000

 67 Following the quality controls used in past RTECS surveys, EIA investigated the possibility that monthly travel 

patterns had changed based on a comparison of estimates between those found in Table B2 and the highway usage 

estimates from the Federal Highway Administration’s Traffic Volume Trends data. The differences were negligible; 

thus, EIA applied the distribution given in Table B2 in order to compute annualized VMT. Some would argue that a 

update of Table B2 is needed; unfortunately, a reasonable travel diary study has not been conducted to EIA’s 

knowledge that would provide such an update. 

ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS 141
APPENDIX B 

Source: 1984 Petroleum Marketing Index (PMI) Survey, NPD Research Inc. The survey is a demographically and 

geographically balanced-quota sample of 4,100 households. Respondents maintained fuel purchase diaries for an 

average of 10 months. As part of the survey, information was collected on the characteristics of trips taken in vehicles

during a designated day. Trip lengths were recorded as respondent perception rather than from odometer readings. The 

distribution of monthly mileage fractions has been obtained from this survey. 

The annual consumption for vehicle i can be thought of as the sum of the individual monthly 

consumptions: 

∑

=

= 12

j 1

Ci cij (9)

where Ci denotes annual consumption of vehicle fuel for the ith vehicle, in gasoline equivalent

gallons and cij denotes consumption of vehicle motor fuel for the ith vehicle during the jth month. 

Because the VMT values – as computed by ORNL and discussed in the NHTS User’s Guide 

– in the January 2004 public-use file provided by NHTS assume each vehicle was available for 

the entire 12-month period of the survey year, consumption and expenditure values for vehicle 

use are over-estimated. To eliminate, where possible, such over-estimation, EIA has provided 

another public-use file in which the annual consumption for vehicle i can be thought of as the 

sum of the monthly consumption values, where the period covered equals the possession time of 

vehicles. Thus, the starting and ending months refer to the possession time of vehicle i by the 

household. 

In EIA’s public-use file, consumption is calculated only over those months that vehicle i was

derived to be owned or used by the household. In this sense, “annual” does not necessarily mean

a full 12-month period. This is an important point since fuel economy varies seasonally. 

Consumption for each month may be expressed in terms of monthly VMT and monthly inuse fuel economy:

ij

ij

ij

mpg

m

c = , ∀ j = 1, 2, ..., 12 (10) 

where mij denotes VMT for the ith vehicle during the jth month and mpgij denotes fuel 

economy in miles per gasoline equivalent gallon for the ith vehicle during the jth month. Now, 

Equation 10 can be rewritten as: 

∑

∈

=

j used mpg

m C

ij

ij

i (11)

ORNL (41,474 vehicles) and EIA (1,262 vehicles) provided the annualized VMT estimate 

(i.e., owned or available for use for the entire year) for NHTS that was used to calculate monthly

VMT values. Given that value, a monthly VMT was derived for each annualized vehicle VMT

as: 

mij Mi f (i, j) = • (12)

142 ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS
APPENDIX B 

where Mi denotes for the ith vehicle, calculated using odometer readings and procedures

discussed in Appendix J and fij denotes the average fraction of “annual” VMT that was driven 

during the jth month, estimate for the ith vehicle. For all sample vehicles, f(i,j) is a function of the 

average fractions, Fj, found in Table B2.

There is no single distribution of average monthly VMT fractions, f(i,j). Rather, there was a

family of distributions, depending on which particular months a vehicle was owned or used by a 

household. Because the monthly VMT fractions for a given vehicle i always sums to one – no

matter the timeframe in which the vehicle was owned or used by the household – the following

identity is always true: 

 

1,2,3, , n

j

f 1, ∑ (i, j) ∀ = K

∈

=

used

(13)

The ith vehicle’s f(i,j) were derived from Fj values found in Table B2 as follows: 

∑

∈

=

j used

F j

F j f (i, j) (14)

If we assume that each and every vehicle is owned or used by its sampled household, then 

substituting mpgij = MPGij(in-use) and mij from Equation 12 into Equation 11 yields the following 

estimate of annual consumption for the ith vehicle: 

∑

=

• = 12

j 1 MPG

M F C

ij(in - use)

i ij

i (15)

The public-use file disseminated by NHTS (January 2004) makes the above assumption on

the timeframe for vehicle use. While the NHTS public-use file provide estimates based on the 

assumption that each and every sample vehicle was present in the sample household for 12 

months, EIA’s created an alternate estimator for consumption, Ci

(EIA)

, in which acquired and 

disposed vehicles during the survey period are accounted for. This estimator is written as: 

∑

∈

• =

j used MPG

M

C(EIA)

i ij(in - use)

i f (i, j) (16)

To simply calculations, a single “annualized” fuel economy, analogous to the “annualized” 

MPGi from previous EIA surveys of the residential transportation sector, was estimated as: 

∑

∈

•

=

j used

f (i, j) GPMR

MPG MPG

ij(in - use)

i(EPA 55/45)

i(annualized) (17)

ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS 143
APPENDIX B 

Thus, annual consumption equals: 

i(annualized)

i

i

MPG

M C = . (18)

ANNUAL VEHICLE FUEL EXPENDITURES AND PRICE 

VEHICLE FUEL EXPENDITURES 

In the 2001 NHTS, fuel expenditures were calculated by multiplying the vehicle-fuel 

consumption by the price of the vehicle fuel. The 2001 NHTS did not collect vehicle fuel prices 

via fuel purchase diaries. Instead, each NHTS sample vehicle was assigned a price based on 

imputed engine type and fuel metering values obtained from the NHTSA Corporate Average Fuel

Economy files for model year’s 1978-2001. For pre-1978 model year vehicles, otto engine and

gasoline were imputed for engine type and fuel metering, respectively. Fuel prices, by month, 

were obtained from the following Energy Information Administration survey questionnaires: 

• Form EIA-782A68 “Refiners'/Gas Plant Operators' Monthly Petroleum Product Sales 

Report.” 

• Form EIA-782B69 “Resellers'/Retailers' Monthly Petroleum Product Sales Report.” 

• Form EIA-88870 “On-Highway Diesel Fuel Price Survey.” 

• Form EIA-89571 “Monthly Quantity and Value of Natural Gas Report.” 

• Form EIA-82672 “Monthly Electric Utility Sales and Revenue Report with State

Distributions.”

It is important to define the transportation fuels included in each of these prices. See the

following sections for further details on transportation fuel prices. 

 68 Price and volume data at a State level for 14 petroleum products for various retail and wholesale marketing 

categories are reported by the universe of refiners and gas plant operators. 

69 Price and volume data at the State level for gasoline, No. 2 distillate, propane, and residual fuel are reported by a

sample of distillate fuel oil resellers and retailers, motor gasoline wholesalers, and residual fuel oil resellers and 

retailers. 

70 The Form EIA-888 survey collects data on the National and Petroleum Administration for Defense (PAD) District 

level cash price of self-serve, motor vehicle diesel fuel. The data are used to monitor changes in motor vehicle diesel 

fuel prices and to report to the Congress and others when requested. Respondents are a scientifically selected sample of

companies owning retail outlets which sell motor vehicle diesel fuel. 

71 Monthly and annual production data are collected from the appropriate agencies of the natural gas producing States. 

72 Form EIA-826 collects information from regulated and unregulated companies that sell or deliver electric power to 

end users, including electric utilities, energy service providers, and distribution companies.. 

144 ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS
APPENDIX B 

It is also important to point out that the NHTS did not collect information on the use of

alternate fuels. Because of that omission, it was not possible to properly assign fuel consumption 

for dual-fuel (or flexible-fuel) vehicles. While these supplemental data do not explicitly account 

for alternative fuel use, the supplemental NHTS data should allow for a user to freely assign an 

alternative fuel use fraction. For example, one common assumption is to assign an operating 

scenario where 50 percent of the time the vehicle runs on alternative fuel (e.g., E85) and 50 

percent of the time on conventional fuel (i.e., gasoline). Using the supplemental data and VMT 

estimate, in conjunction with EIA’s fuel economy adjustment methodology, a user may make

their own assignment of alternative fuel use. Because allowances have been made for selfestimating alternate fuel use and, more importantly, the NHTS collected no data to verify any 

method for assigning alternative fuel use, all consumption and expenditures supplemental data are 

based on a dedicated use of motor gasoline, diesel, natural gas, or electricity. That is, all flexiblefuel vehicles are assumed to operate on 100 percent gasoline. Thus, estimates for flexible-fuel 

vehicles are accurate to the extent that this assumption is valid. 

Unfortunately, respondents were not asked the type of fuel purchased for their transportation 

demands. Further, respondents were not queried on the grade of their purchased fuels. Thus, fuel 

type was imputed to a sample vehicle based on its representative “match” with the selected

vehicle from the NHTSA files. A matching record was chosen from among the several applicable 

ones, with probability proportional to sales, using the sales figures on the NHTSA files. Once 

chosen, a record provided (1) EPA Composite MPG, (2) fuel metering, and (3) engine type. The

later two items provided enough information to impute a fuel type to a “matched” sample vehicle. 

The EIA price series are published by month, by State, 5 PAD districts (PADD), and by type 

and grade of fuel. For the 2001 NHTS, annual fuel expenditures, Ei, was estimated by multiplying

monthly gasoline prices by monthly consumption to produce monthly expenditures, summing

over the monthly expenditures derived annual expenditures. 

TYPE OF FUEL USED

Table B3 provides the percentage distribution of RTECS vehicles by fuel type categories. In

1994, the latest year for which RTECS estimates are available, 97.9 percent of the 156.8 million 

RTECS vehicles used gasoline. The remaining 2.1 percent of vehicles used diesel fuel or other 

fuel types. 

Table B3. Distribution of Residential Transportation Energy Consumption Survey Vehicles by Type 

of Fuel Used, 1994 

Type of Vehicle Fuel Number of Vehicles Percent of Vehicles 

Total 156.8 100.0

Gasoline 153.4 97.9

 Leaded Q Q

 Unleaded 151.5 96.7

 Regular 14.2 66.4

 Premium 26.7 17.1

 Intermediate 20.6 13.2

ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS 145
APPENDIX B 

Table B3. Distribution of Residential Transportation Energy Consumption Survey Vehicles by Type 

of Fuel Used, 1994 

Type of Vehicle Fuel Number of Vehicles Percent of Vehicles 

Diesel 1.8 1.1

Gasohol 1.4 0.9

Notes: Because of rounding, data may not sum to totals. Q = Data withheld either because the Relative Standard Error 

(RSE) was greater than 50 percent or fewer than 10 households were sampled. Source: Energy Information

Administration, Office of Energy Markets and End Use, 1994 Residential Transportation Energy Consumption Survey. 

Comparing the 1994 RTECS and augmented 2001 NHTS estimates is not advisable because 

of the differences in data collected by the RTECS and the imputed information for the NHTS. 

Unfortunately, no fuel metering or fuel type information was collected on the NHTS for such a 

comparison. 

GASOLINE PRICES 

Prices published by the EIA supplier surveys are pre-tax prices for conventional, oxygenated,

and reformulated motor gasoline. Pre-tax prices were supplemented with Federal and State tax

rates, by month, to derive retail motor gasoline prices; information on tax rates for gasoline are 

available from the Federal Highway Administration's web site. These pre-tax prices are 

published monthly, by State, in EIA’s Petroleum Marketing Monthly, which includes price 

(excluding taxes) and volume data at a State level for 14 petroleum products for various retail and

wholesale marketing categories are reported by the universe of refiners and gas plant operators. 

Because the NHTS did not collect the type or grade of gasoline consumed in each sample

vehicle, gasoline price was assigned a monthly fuel price that represents a State’s volumeweighted average of gasoline. The below is an excerpt from the glossary of the Petroleum 

Marketing Monthly, as reported by EIA, which identifies the composition of the motor gasoline

prices used in this appendix. 

Motor Gasoline (Finished): A complex mixture of relatively volatile hydrocarbons with or

without small quantities of additives, blended to form a fuel suitable for use in sparkignition engines. Motor gasoline, as defined in ASTM Specification D-4814 or Federal

Specification VV-G-1690B, is characterized as having a boiling range of 122 to 158 

degrees Fahrenheit at the 10 percent recovery point to 365 to 374 degrees Fahrenheit at 

the 90 percent recovery point. “Motor Gasoline” includes conventional gasoline; all types 

of oxygenated gasoline, including gasohol; and reformulated gasoline, but excludes 

aviation gasoline. 

Conventional Gasoline: Motor gasoline not included in the oxygenated or reformulated

gasoline categories. Excludes reformulated gasoline blendstock for oxygenate blending 

(RBOB). 

146 ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS
APPENDIX B 

Oxygenated Gasoline: Finished motor gasoline, other than reformulated gasoline, having 

an oxygen content of 2.7 percent or higher by weight and required by the U.S. 

Environmental Protection Agency (EPA) to be sold in areas designated by EPA as carbon

monoxide (CO) nonattainment areas. Note: Oxygenated gasoline excludes oxygenated 

fuels program reformulated gasoline (OPRG) and reformulated gasoline blendstock for 

oxygenate blending (RBOB). Data on gasohol that has at least 2.7 percent oxygen, by 

weight, and is intended for sale inside CO nonattainment areas are included in data on 

oxygenated gasoline. Other data on gasohol are included in data on conventional

gasoline. 

Reformulated Gasoline: Finished motor gasoline formulated for use in motor vehicles, the 

composition and properties of which meet the requirements of the reformulated gasoline 

regulations promulgated by the U.S. Environmental Protection Agency under Section 

211(k) of the Clean Air Act. Note: This category includes oxygenated fuels program 

reformulated gasoline (OPRG) but excludes reformulated gasoline blendstock for

oxygenate blending (RBOB). 

Further, EIA classifies gasoline by octane ratings, where each type of gasoline (conventional,

oxygenated, and reformulated) is classified by three grades: 

1) Regular Gasoline: Gasoline having an antiknock index (i.e., octane rating)

greater than or equal to 85 and less than 88. Note: Octane requirements may vary 

by altitude. 

2) Midgrade Gasoline: Gasoline having an antiknock index (i.e., octane rating) 

greater than or equal to 88 and less than or equal to 90. Note: Octane 

requirements may vary by altitude. 

3) Premium Gasoline: Gasoline having an antiknock index (i.e., octane rating) 

greater than 90. Note: Octane requirements may vary by altitude. 

ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS 147
APPENDIX B 

Figure B3. Area Map for Reformulated Gasoline 

Source: Energy Information Administration, website 

www.eia.doe.gov/oil_gas/petroleum/data_publications/wrgp/reformulated_map.html. 

DIESEL FUEL PRICES 

Prices published by the EIA supplier surveys are at the retail level for diesel fuel. The form

EIA-888 survey collects data on the National and Petroleum Administration for Defense (PAD) 

District73 level cash price of self-serve, motor vehicle diesel fuel. The data are used to monitor 

changes in motor vehicle diesel fuel prices and to report to the Congress and others when

requested. Respondents are a scientifically selected sample of companies owning retail outlets 

that sell motor vehicle diesel fuel. Prices are published on 

http://tonto.eia.doe.gov/oog/info/wohdp/diesel.asp by EIA. 

EIA conducts weekly Computer Assisted Telephone Interview surveys that collect prices at 

the outlet level. The EIA-888 collects prices of diesel fuel from truck stops and service stations 

across the country each Monday morning. Average prices of diesel fuel through outlets at the 

five Petroleum Administration for Defense District (PADD) levels, regions of the country, subPADD levels, and the state of California are released by the end of the day through Listserv, the 

Web, Fax, and telephone hotline. 

Because the NHTS did not collect the type or grade of diesel consumed in each sample

vehicle, diesel price was assigned to a diesel-powered vehicle based on a monthly fuel price 

 73 PAD District 1 (East Coast) is composed of the following three subdistricts: Subdistrict 1A (New England):

Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, Vermont. Subdistrict 1B (Central Atlantic): 

Delaware, District of Columbia, Maryland, New Jersey, New York, Pennsylvania. Subdistrict 1C (Lower Atlantic): 

Florida, Georgia, North Carolina, South Carolina, Virginia, West Virginia. PAD District 2 (Midwest): Illinois, Indiana, 

Iowa, Kansas, Kentucky, Michigan, Minnesota, Missouri, Nebraska, North Dakota, South Dakota, Ohio, Oklahoma, 

Tennessee, Wisconsin. PAD District 3 (Gulf Coast): Alabama, Arkansas, Louisiana, Mississippi, New Mexico, Texas.

PAD District 4 (Rocky Mountain): Colorado Idaho, Montana, Utah, Wyoming. PAD District 5 (West Coast): Alaska, 

Arizona, California, Hawaii, Nevada, Oregon, and Washington. 

148 ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS
APPENDIX B 

represented by a PAD that includes the State in which the sample vehicle resides, according to 

NHTS, with the notable exception of the state of California where assignment was completed 

within state geographic boundaries. 

Figure B4. Map of Petroleum Administration for Defense Districts 

OTHER FUEL TYPE PRICES

According to the 1994 RTECS, approximately 1.4 million vehicles were reported using 

gasohol. Unfortunately, in the 2001 NHTS, there is no comparable statistic because alternativefuel vehicles were imputed as dedicated gasoline vehicles. That imputation rule was applied 

because (1) NHTS did not collect fuel type information on its survey questionnaire and (2) the

majority of owners of vehicles capable of being powered by methanol, ethanol, and other 

alternative fuels are consuming motor gasoline since alternative fueling stations do not serve 

large areas of the nation.74

While the NHTS cannot delineate gasohol use, this appendix does address dedicated

compressed natural gas (CNG) and electric vehicles.75 For CNG, retail prices were obtained from

form EIA-895, “Monthly Quantity and Value of Natural Gas Report”. The EIA-895 collects 

monthly information from the applicable State agencies that collect data concerning natural gas 

production. Data are published in several of EIA's monthly and annual reports. For electricity,

retail prices were obtained from form EIA-826, “Monthly Electric Utility Sales and Revenue

Report with State Distributions.” Form EIA-826 collects information from regulated and

unregulated companies that sell or deliver electric power to end users. While three customer

groups were available, residential customers were selected to represent electric prices because 

 74 U.S. Department of Energy, Energy Efficiency and Renewable Energy, Alternative Fuels Data Center reports that

nearly 5 thousand fueling stations dispense alternative fuels (see listing details on 

http://www.eere.energy.gov/afdc/infrastructure/station_counts.html, accessed March 9, 2005). 

75 Propane vehicles are not included in the NHTSA files. Thus, no propane-fuel vehicles are found in the additions 

made by EIA to NHTS data.

ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS 149
APPENDIX B 

this group most accurately reflected the retail electric price for NHTS households. State and

regional summaries of these data are published by EIA and used by public and private analysts. 

150 ENERGY INFORMATION ADMINISTRATION/HOUSEHOLD VEHICLES ENERGY USE: LATEST DATA & TRENDS