Posted on Thursday, March 05, 2009 - 11:14 am:

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How Vehicle Testing Is Changed

Since the EPA began gathering mileage ratings in the 1970s, it has not provided figures derived from real-world driving.

Instead, the numbers stem from laboratory testing required by law to measure vehicle emissions. Basically, government officials measure the amount of carbon expelled in a vehicle's exhaust during testing to deduct how much fuel has been consumed. (This system came about after auto companies complained about the cost and burden of having to do an additional test just for fuel economy.)

During testing, new vehicles—usually pre-production models—are "driven" on a dynamometer so wheels rotate as they would on a road. But there's no wind or other weather conditions in the laboratory.

There also are no changes in road grade, and the test driver must follow a strict "schedule" of accelerator and speed situations that are meant to mimic real-world driving.

The results are then put into a complicated formula to derive the mileage artificially. This formula was adjusted once before—in the 1980s—to decrease mileage figures and better reflect real driving conditions.

This time, the EPA is adding new testing to account for drivers accelerating more quickly than in previous tests, driving at higher speeds and using air conditioning. It also is taking into account lower outside temperatures. All these factors can lower a vehicle's fuel economy.

Note, however, that even the new EPA figures are estimates and do not necessarily indicate the exact mileage every driver will get.

Mileage is affected by driver habits, weather, the use of power-draining accessories, terrain, the condition of a vehicle, even tire pressure.

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