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About Fuel Economy Ratings
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 Why is Fuel Economy Important?
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Header: How Vehicles Are Tested

Fuel economy is measured under controlled conditions in a laboratory using a standardized test procedure specified by federal law. Manufacturers test their own vehicles—usually pre-production prototypes—and report the results to EPA. EPA reviews the results and confirms about 10-15 percent of them through their own tests at the National Vehicles and Fuel Emissions Laboratory.

In the laboratory, the vehicle's drive wheels are placed on a machine called a dynamometer that simulates the driving environment—much like an exercise bike simulates cycling.

The energy required to move the rollers can be adjusted to account for aerodynamic forces and the vehicle's weight.

 Photo: Car on dynamometer

On the dynamometer, a professional driver runs the vehicle through a standardized driving routine, or schedule, which simulates “typical” trips in the city or on the highway.

See video

 Photo: Driver running car through test cycle on dynamometer

Each schedule specifies the speed the vehicle must travel during each second in the test.

Right: The driver watches a computerized display that shows his driving statistics compared to the specified schedule.

See video

 Photo: Driver watching computerized display showing test schedule and driving statistics

A hose is connected to the tailpipe to collect the engine exhaust.

The amount of carbon in the exhaust is measured to calculate the amount of fuel used during the test.

This is more accurate than using a fuel gauge.

 Photo: Hose connected to vehicle tailpiple; sending emissions to test equipment

Adjusting Estimates

In the 1980s, an EPA study found that drivers were typically achieving lower fuel economy than predicted by EPA laboratory tests. As a result, EPA required the laboratory-derived city and highway MPG estimates posted on the labels of new vehicles to be adjusted downward by 10 percent for city estimates and by 22 percent for highway estimates to better reflect the MPG real-world drivers can expect.