1

Emission

Facts

Office of Transportation

and Air Quality

EPA420-F-05-004

February 2005

Greenhouse Gas Emissions from a

Typical Passenger Vehicle

The U.S. Environmental Protection Agency (EPA) developed this series

of four fact sheets to facilitate consistency of assumptions and practices

in the calculation of emissions of greenhouse gases from transportation

and mobile sources. They are intended as a reference for anyone

estimating emissions benefits of mobile sources air pollution control

programs.

Issue

Each EPA voluntary climate change program has used slightly different

assumptions to translate the greenhouse gas (GHG) reductions associated with the program to the equivalent GHG emissions of a number of

cars on the road. The result is that different numbers for the greenhouse

gas emissions associated with a passenger vehicle have been used for

different programs. The purpose of this fact sheet is to determine consistent assumptions and produce a number that is accepted for the annual

GHG emissions associated with a passenger vehicle. The estimate

calculated here is for vehicle emissions only, and does not include

lifecycle emissions such as emissions associated with the production and

distribution of fuel.
2

Recommendation

To translate GHG reductions into an equivalent number of cars off the

road, annual emissions from a typical passenger vehicle should be equated

to 5.5 metric tons of carbon dioxide equivalent or 1.5 metric tons of carbon

equivalent.

Key steps to the calculation

There are six key steps to estimate the annual greenhouse gas emissions

associated with a passenger vehicle:

1. Determining the carbon dioxide (CO2) produced per gallon of

gasoline

2. Estimating the fuel economy of passenger cars and light trucks (in

miles per gallon [mpg])

3. Determining the number of miles driven

4. Determining the emissions of greenhouse gases other than CO2

(methane [CH4], nitrous oxide [N2O], and hydrofluorocarbons

[HFCs])

5. Estimating the relative percentages of passenger cars and light trucks

6. Calculating the resulting annual greenhouse gas emissions

Note that for the purposes of this fact sheet, representative values were

chosen for each of these variables, despite the fact that in practice variation

does occur in these numbers.

A gallon of gasoline is assumed to produce 8.8 kilograms (or 19.4 pounds)

of CO2

. This number is calculated from values in the Code of Federal

Regulations at 40 CFR 600.113-78, which EPA uses to calculate the fuel

economy of vehicles, and relies on assumptions consistent with the Intergovernmental Panel on Climate Change (IPCC) guidelines.

In particular, 40 CFR 600.113-78 gives a carbon content value of 2,421

grams (g) of carbon per gallon of gasoline, which produces 8,877 g of CO2

.

(The carbon content is multiplied by the ratio of the molecular weight of

CO2

 to the molecular weight of carbon: 44/12).

Step 1:

Determining

the CO2

produced per

gallon of

gasoline
3

This number is then multiplied by an oxidation factor of 0.99, which

assumes that 1 percent of the carbon remains un-oxidized.1

 This produces

a value of 8,788 g or 8.8 kg (19.4 lbs) of CO2.

There are two sources of data which EPA has used for the average fuel

economy of passenger cars and light trucks. MOBILE6.2 (EPA’s computer model for estimating emissions for highway vehicles) can calculate

an average fuel economy across the fleet, based on the EPA annual Fuel

Economy Trends reports. For 2003, MOBILE calculates values of 23.9

miles per gallon (mpg) for passenger cars and 17.4 mpg for light trucks.

These values are weighted averages (based on vehicle age data for the

fleet, including vehicles up to 25 years old) of the Fuel Economy Trends

sales-weighted average fuel economy of passenger cars and light trucks

for each model year. MOBILE6.2 calculates an overall average fuel

economy for passenger vehicles of 20.3 mpg (weighted by vehicle miles

traveled [VMT] for passenger cars and light trucks).

The Federal Highway Administration’s (FHWA) "Highway Statistics

2001" gives average values of 22.1 mpg for passenger cars and 17.6 mpg

for light trucks as a fleet wide average in for the year 2001 (includes all

vehicles on the road in 2001). These values are obtained by dividing

vehicle miles traveled by fuel use.2

 These values are used in the development of the "Inventory of U.S. Greenhouse Gas Emissions and Sinks."

Recommendation: Values were calculated using both sets of fuel

economy numbers. Depending on the circumstances, use of one set of

numbers or the other may be more appropriate. Generally EPA staff

should use the MOBILE6 estimates. However, EPA uses the FHWA

numbers in developing the National Inventory for Greenhouse Gas

Emissions because they are consistent with the methodology used to

develop the inventory. (Note that a small variation in the fuel economy

number will not change the rough estimate of greenhouse gases derived

here.)

1

 The International Panel on Climate Change Guidelines (IPCC) recommends a

fraction of carbon oxidized factor of 0.99 for all oil and oil-based products. Based on

the fundamentals of internal combustion engine design and combustion, EPA is

currently examining whether this fraction is higher (closer to 100 percent) for gasoline

vehicles in the US.

2

 U.S. Department of Transportation, Federal Highway Administration, "Highway

Statistics 2000," Washington, DC, 2001. Vehicle travel and fuel use data are kept

separately for passenger cars and light trucks.

Step 2:

Estimating the

fuel economy

of passenger

cars and light

trucks (MPG

estimate)
4

The number of miles driven per year is assumed to be 12,000 miles for

all passenger vehicles.

This number is based on several sources. Calculations from EPA’s

MOBILE6 model show an average annual milage of roughly 10,500

miles per year for passenger cars and over 12,400 miles per year for light

trucks across all vehicles in the fleet. However, these numbers include

the oldest vehicles in the fleet (vehicles 25 years of age and older), which

are likely not used as primary vehicles and are driven substantially less

than newer vehicles. Since this calculation is for a typical vehicle,

including the oldest vehicles may not be appropriate. For all vehicles up

to 10 years old, MOBILE6 shows an annual average milage of close to

12,000 miles per year for passenger cars, and over 15,000 miles per year

for light trucks.

FHWA’s National Highway Statistics contains values of 11,766 miles for

passenger cars and 11,140 miles for light trucks across the fleet. However, as with the MOBILE6 fleet-wide estimates, these numbers include

the oldest vehicles in the fleet. EPA’s Commuter Model uses 1997 data

from Oak Ridge Laboratories for the number of cars nationally and

number of miles driven which produces a value of just over 12,000 miles

per year. Due to the wide range of estimates, 12,000 miles per vehicle is

used as a rough estimate for calculating the greenhouse gas emissions

from a typical passenger vehicle.)

In addition to carbon dioxide, automobiles produce methane (CH4) and

nitrous oxide (N2O) from the tailpipe, as well as HFC emissions from

leaking air conditioners.

The emissions of CH4 and N2O are related to vehicle miles traveled

rather than fuel consumption, and the emissions of CH4

, N2O, and HFCs

are not as easily estimated from a vehicle as for CO2.

3

 On average, CH4,

N2O, and HFC emissions represent roughly 5 - 6 percent of the GHG

emissions from passenger vehicles, while CO2

 emissions account for 94-

95 percent, accounting for the global warming potential of each greenhouse gas. (These percentages are estimated from the EPA "Inventory of

U.S. Greenhouse Gas Emissions and Sinks: 1990 - 2001.") To simplify

this estimate, it is assumed that CH4, N2O, and HFCs account for 5

percent of emissions, and the CO2 estimate was multiplied by 100/95 to

incorporate the contribution of the other greenhouse gases.

3

 EPA is currently examining ways to better disaggregate the HFC emissions from

vehicles.

Step 3:

Determining

the number of

miles driven

Step 4:

Determining

the emissions

of greenhouse

gases other

than CO2

 (N2O,

CH4, and

HFCs)
5

Because FHWA calculates fuel economy for passenger cars and light

trucks separately, it is necessary to determine the relative percentage of

cars and light trucks in order to derive the greenhouse gas emissions for

an average passenger vehicle. (This step is not necessary when using the

MOBILE6 fuel economy data because MOBILE6 already calculates a

weighted average fuel economy for all passenger vehicles.) Passenger

cars are assumed to make up 63.4 percent and light trucks make up 36.6

percent of the passenger vehicle fleet. These values are derived from

table 6.4 (2000 data) of the "Transportation Energy Data Book: Edition

22" (published by the Center for Transportation Analysis, Oak Ridge

National Laboratory), which states there are 127,721,000 passenger cars

on the road and 73,775,000 light trucks (less than 8500 lbs4

). Note that

this percentage is changing over time, as light trucks now represent

roughly 50 percent of annual new vehicle sales.

A: Using EPA MOBILE6.2 fuel economy numbers

Metric tons of CO2e for the average passenger vehicle =

(VMT/passenger vehicle avg. MPG) x CO2 per gallon x (100/95) /1000 =

(12,000/20.3) x 8.8 x (100/95)/1000 =

5.48 metric tons CO2e for the average passenger vehicle (1.49 metric

tons CE)

B: Using DOT fuel economy numbers

[%LDV x (LDVVMT/LDVMPG) x CO2 per gallon x (100/95) /1000] +

[%LDT x (LDTVMT/LDTMPG) x CO2 per gallon x (100/95) /1000] =

[0.634 x (12,000/22.1) x 8.8 x (100/95)/1000] + [0.366 x (12,000/17.6)]

x 8.8 x (100/95)/1000] =

5.03 metric tons CO2

e for passenger cars and 6.32 metric tons CO2e for

light trucks (= 1.37 metric tons CE for cars and 1.72 metric tons CE for

trucks) =

5.50 metric tons CO2e for the average passenger vehicle (1.50 metric

tons CE)

4

 Vehicles over 8500 lbs are often not included in the light truck category. These

vehicles are not required to meet CAFE standards. Examples of these vehicles include

the Hummer and the Ford Excursion.

Step 5:

Estimating the

relative

percentages

of passenger

cars and light

trucks

Step 6:

Calculating

the resulting

annual

greenhouse

gases from a

typical

passenger

vehicle
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Recommendation: To calculate rough translations of GHG reductions

into an equivalent number of cars off the road, use 5.5 metric tons of

CO2

, or 1.5 metric tons of carbon equivalent. This number is rounded to

the nearest tenth of a ton (using either DOT or EPA fuel economy

estimates). This rough estimate will also allow for some variability in

the underlying variables.

A: Using EPA MOBILE6.2 fuel economy numbers

Average passengerr vehicle = 5.20 metric tons CO2

e (1.42 metric tons

CE)

B: Using DOT fuel economy numbers

Passenger Cars = 4.78 metric tons CO2

e (1.30 metric tons CE)

Light Trucks = 6.00 metric tons CO2e (1.64 metric tons CE)

All passenger vehicles = 5.23 metric tons CO2

e (1.43 metric tons CE)

Recommendation: For CO2

 only estimate, use 5.2 metric tons CO2e, or

1.4 metric tons CE

Note: These calculations and the supporting data have associated

variation and uncertainty. EPA may use other values in certain circumstances, and in some cases it may be appropriate to use a range of

values.

For More Information

You can access documents on greenhouse gas emissions on the Office of

Transportation and Air Quality Web site at:

www.epa.gov/otaq/greenhousegases.htm

For further information on calculating emissions of greenhouse gases,

please contact Ed Coe at:

U. S. Environmental Protection Agency

Office of Transportation and Air Quality

1200 Pennsylvania Ave., NW (6406J)

Washington, DC 20460

202-343-9629

E-mail: coe.edmund@epa.gov

CO2

 only

numbers