Cars offered for sale often come with a rating indicating how many miles per gallon they get. Fuel efficiency is important to many drivers, and it is sometimes used to determine additional taxes and fees which may need to be paid, such as the gas guzzler tax. In the United States, these mileage ratings are determined by the Environmental Protection Agency (EPA), which also makes test data public for people who are interested in more detailed information.
Not every car is subjected to miles per gallon tests by the EPA. Car manufacturers are mandated to test representative vehicles of a general class, rather than each available model; in other words, each version of a Honda Civic is not tested, because it is assumed that the vehicles have similar levels of fuel economy. Motorcycles and very large trucks are exempt from this testing, although some manufacturers may choose to test them anyway. Manufacturers self report the results of their testing to the EPA, which may decide to run additional tests on its own.
As of 2008, there will be three basic tests or schedules used to determine fuel economy. The first simulates city driving conditions, in which a car is started with a cold engine and stopped and started multiple times. The second tests out miles per gallon in highway situations, where cars travel at a consistent rate of speed with warmed engines. Finally, cars will be tested in conditions with controlled temperatures including hot weather, neutral temperatures, and cold weather to see how these variables impact fuel economy.
After testing, the EPA adjusts the miles per gallon reading it gets to reflect real world conditions outside the laboratory. It also comes up with a weighted average, combining 55% city mileage and 45% highway mileage for a single number. The EPA testing isn't perfect, but the temperature controlled tests are one effort to make the EPA miles per gallon readings more accurate, by testing cars in a variety of situations to see how they perform.
Many drivers like to track their own fuel economy by keeping track of how many miles they can go on a single tank of gas. Drivers may have noticed that their fuel economy is not always exactly the same as the EPA predictions for their vehicles, because variables like loaded weight, tire condition, weather, temperature, and gas quality can all influence fuel efficiency. If drivers notice a sudden downturn in fuel economy, they may want to consider taking the car to the shop to see what's wrong, or checking to make sure that their gas station is pumping as much gas as they claim to be. Most regions have a regulatory agency which inspects gas stations of a regular basis to check for this type of fraud.