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How the 2008 EPA Fuel Economy Ratings Work

Comparing the Fuel Economy Tests
It was time for fuel efficiency tests to get a tune-up.
It was time for fuel efficiency tests to get a tune-up.
Mervyn Penrose Rands/Getty Images

The original test used to estimate a car's fuel economy was based on everyday driving in the 1960s, and the last update to the test was in 1985. That means it was geared toward slower acceleration, slower highway speeds, and few in-car accessories that draw power from the engine. The test made a lot of sense at the time, but as cars have changed, the test needed to as well.

The test itself is performed on a dynamometer, which allows testers to control the testing environment and get accurate readouts because the car can be attached to sensors. The original test on the dynamometer mimicked everyday driving by taking the car through simulated stop-and-go traffic and highway driving. The test results were then multiplied by a set number, to account for things like wind resistance.­

The new test is very similar to the old test. It still takes place on a dynamometer and still multiplies the results by a set number to take wind resistance into account and get the final fuel economy estimate. What's changed are the driving conditions that the dynamometer mimics. Now the test uses faster acceleration and highway speeds. It also accounts for the use of air conditioning. Finally, the test mimics a start in 20 degree cold (the colder the weather, the more gas it takes to start a car). With these changes, the new test now more closely resembles the type of driving most people do.

So now you know how the test has changed, but what does it mean for you? We'll explain that on the next page.