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The Environmental Protection Agency (EPA) rates cars for miles per gallon, but not every model is tested. Automakers test representative vehicles and report the results to the EPA. The EPA adjusts the rating to reflect real-world conditions and uses a weighted average. Drivers may track their own fuel economy, but variables can influence it.
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 is sometimes used to determine additional taxes and fees that may be paid, such as excise fuel tax. In the United States, these mileage ratings are determined by the Environmental Protection Agency (EPA), which also makes test data public for those interested in more detailed information.
The EPA does not rate all cars for miles per gallon. Automakers are required to test vehicles representative of a general class, rather than every model available; in other words, each version of a Honda Civic is not tested, because the vehicles are assumed to have similar levels of fuel economy. Motorcycles and very large trucks are exempt from this test, although some manufacturers may choose to test them anyway. Manufacturers report the results of their tests to the EPA, which may decide to conduct additional tests at its own expense.
Beginning in 2008, there will be three basic tests or schedules used to determine fuel economy. The first simulates city driving conditions, where a car starts with a cold engine and stops and starts several times. The second tests miles per gallon in highway situations, where cars are traveling at a constant speed with warmed up engines. Finally, the cars will be tested in temperature controlled conditions 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 to reflect real-world conditions outside of the laboratory. It also features a weighted average, which combines 55% city mileage and 45% highway mileage for a single number. EPA testing isn’t perfect, but temperature-controlled testing is an effort to make EPA mpg readings more accurate, 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 such as loaded weight, tire condition, weather, temperature, and gas quality can influence fuel efficiency. If drivers notice a sudden drop in fuel economy, they may want to consider taking the car to the shop to see what’s going on, or check to make sure their gas station is pumping out as much gas as they claim. Most regions have a regulatory agency that inspects service stations on a regular basis to check for this type of fraud.
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