Miles per gallon (mpg) is the measurement of the distance in miles that a vehicle can travel using only one gallon of fuel. The measurement is used primarily to demonstrate the fuel efficiency of an automobile, with those capable of a higher mpg rating providing a savings in operational costs.
The U.S. Environmental Protection Agency (EPA) is the government body that certifies mpg figures.
Getting an accurate and consistent measurement of a vehicle's mpg can be difficult. The actual amount can be affected by a number of factors, including road and traffic conditions. Because of the way these differing variables can alter the ratings, the EPA runs mpg tests over selected courses, with multiple test runs made to get a statistical average. There are also generally three mpg ratings determined for any given vehicle: the city mpg, highway mpg and a combined, weighted average of the two.
Highway mpg is a measurement of the mpg that a driver will average while driving on an open stretch of road. This score reflects driving conditions that include high-speed operation without stopping during the gallon's use. The city mpg score represents very different driving conditions than those of the highway test, with an emphasis on the short stop-and-start trips often made during in-city operations.
With the growing popularity of alternatively fueled vehicles, EPA testing needed to be expanded to include electricity, and an energy equivalency calculation was needed. This allowed testers to run each vehicle through the mpg tests with the same amount of energy available for the vehicle's engine as that put out by a gallon of gas. With the equivalency system, an electric car could be fairly compared to a gasoline-powered car, for example, at least in terms of fuel efficiency.
With extensive testing numbers available, mpg can provide consumers with a comparison point that can help them choose between not only a variety of car models, but also a variety of fuel types as well. A car's mpg can make a big difference in the cost of running a vehicle, which is of increasing importance whenever fuel prices are high.