The Environmental Protection Agency (EPA) in the U.S. uses several methods to calculate emissions statistics. The first consideration is the amount of carbon dioxide (CO2) produced for each gallon of gasoline. Another consideration the EPA includes in the emissions statistics is the fuel economy of cars and light trucks. In addition, the number of miles driven by vehicles is determined. Finally, the emission statistics include the effects of other greenhouse gases. Other agencies and governments around the world may employ similar or alternative methodologies to calculate emissions statistics.
Greenhouse gases consists of CO2, methane (CH4), nitrous oxide (N2O), and hydrofluorocarbons (HCFs). It is generally accepted by those who believe in man-made climate change that the biggest contributor to global warming and the depletion of the ozone layer is the emission of CO2 from cars, trucks, and factories. Other greenhouse gases such as CH4, N2O, and HCFs also contribute to the emissions statistics. They usually are emitted from the tailpipes of automobiles and from leaking air conditioners.
CO2 is believed to contribute about 95 percent of all greenhouse gases and thus is the primary focus of emissions statistics. A gallon of gasoline usually produces about 19.4 pounds (8.8 kg) of CO2. This calculation is based on the carbon content value of a gallon of gasoline as stated in the U.S. Code of Federal Regulations.
Once the amount of carbon produced from a gallon of gasoline is known, it is necessary to determine how many miles a car can travel on one gallon of gasoline. This is commonly known as the fuel economy and is represented by the unit miles per gallon. The EPA generates an estimated fuel economy for both city driving and highway driving.
The third component to the emissions statistics regarding CO2 emissions is the number of miles cars and trucks will travel in a year. It is generally assumed that all passenger vehicles will travel about 12,000 miles per year. Light trucks are estimated to travel about 15,000 miles per year.
A final calculation to determine the CO2 emission from a passenger car is a bit complex. It utilizes the number of miles driven by the vehicle and divides it by the average fuel economy of a passenger vehicle. This number is then multiplied by the CO2 per gallon. The resulting value is multiplied by 100 and divided by 95. Finally, this value is then divided by 1000.
The average passenger car will emit 5.48 metric tons of CO2 per year. It assumes the car travels 12,000 miles and has a fuel economy of 20.3 mpg. It also uses 8.8 kg of CO2 in the calculation.