«*EPA
Un.tud SwtL-s Office of Transportation EPA420-F-05-004
AuoncC""""1''' P'"tl>rt "" and Air Quality February 2005
Emission
Facts
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 associ-
ated 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 consis-
tent 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.
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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.
1:
Determining
the CO2
produced per
gallon of
gasoline
Key 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 [CHJ, 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 Inter-
governmental Panel on Climate Change (TPCC) 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).
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2:
Estimating the
fuel economy
of passenger
cars and light
trucks (WPG
estimate)
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 Ibs) of CO2.
There are two sources of data which EPA has used for the average fuel
economy of passenger cars and light trucks. MOBELE6.2 (EPA's com-
puter 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 develop-
ment 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.
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Step 3:
Determining
the number of
miles driven
4:
Determining
the emissions
of greenhouse
other
than CO2 (N2O,
CH4, and
MFCs)
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. How-
ever, 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 green-
house 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.
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Step 5:
Estimating the
relative
percentages
of passenger
cars and light
trucks
6:
Calculating
the resulting
annual
greenhouse
from a
typical
passenger
vehicle
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 Ibs4). 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) 71000 =
(12,000/20.3) x 8.8 x (100/95)71000 =
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) 71000] +
[%LDT x (LDTVMT/LDTMPG) x CO2 per gallon x (100/95) 71000] =
[0.634 x (12,000/22.1) x 8.8 x (100/95)71000] + [0.366 x (12,000/17.6)]
x8.8x(100/95)/1000] =
5.03 metric tons CO2e 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 Ibs 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.
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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.
CO2 only A: Using EPA MOBILE6.2 fuel economy numbers
numbers
Average passengerr vehicle = 5.20 metric tons CO2e (1.42 metric tons
CE)
B: Using DOT fuel economy numbers
Passenger Cars = 4.78 metric tons CO2e (1.30 metric tons CE)
Light Trucks = 6.00 metric tons CO2e (1.64 metric tons CE)
All passenger vehicles = 5.23 metric tons CO2e (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 circum-
stances, and in some cases it may be appropriate to use a range of
values.
For 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
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