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How Does the EPA Test and Calculate Fuel Economy?

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fuel economy calculations tests EPA process mpg

When automakers release new vehicles and advertise their performance capabilities and capacities, the numbers always include fuel economy rates. These “miles per gallon” numbers show, obviously, about how many miles this car can travel on a tank of gas. These rates show how efficient the vehicle is compared to other models on the market.

How does the Environmental Protection Agency calculate fuel economy numbers and what do they actually mean?

How the EPA determines average fuel economy for new cars

The EPA has the responsibility of overseeing the rate of fuel consumption (known as the “fuel economy”) displayed on the window sticker of every new car on the market. Having only one organization in charge of testing and calculating rates ensures that consistent methods and measurements are used for every model — which certainly wouldn’t happen if every automaker were in charge of their own fuel economy calculations.

According to the U.S. Department of Energy, automakers test their vehicles in a controlled laboratory in Ann Arbor, Michigan, following government-outlined procedures. The vehicles sit on a dynamometer inside the lab. The operator in the driver’s seat spins the wheels using the dynamometer and runs the vehicle as if it were traveling in motion along a road. A hose inserted in the tailpipe measures the carbon emissions to calculate how much fuel is being burned during these exercises.

Once the automaker reports these results to the EPA, the government organization can choose to review the results with their own tests, which they do about 15-20 percent of the time. Once the data meets corporate average fuel economy standards, the estimated average in-city, in-highway, and combined city/highway driving fuel economy rates are announced. The city rate reflects stop-and-go, lower speed driving; the highway rate reflects high-speed, rarely-stopped driving; and the combined rate is a mixture of both types.

Every decade or so, the EPA updates its test measurements and conditions to reflect the changing technologies, public driving behavior, tires, and environmental conditions in the real world.

Sources: U.S. Department of Energy, U.S. Environmental Protection Agency