The EPA tests vehicles on a dynamometer using two drive cycles. The first, Federal Test Procedure 1975, is run at a low-speed for 41 minutes with a 10-minute break. Speeds average 21 mph (34 km/h) and top out at 57 mph (92 km/h).

The second, higher-speed Highway Fuel Economy Test maxes out at 60 mph (97 km/h) with an average 48 mph (77 km/h). Neither test is run while using accessories, such as air conditioning.

Test vehicles are fueled with ethanol-free gasoline, which is not sold in the U.S. Gasoline sold at the pump typically contains about 10% ethanol, which can reduce a vehicle’s mileage.

The EPA tests 15% of all new models. The rest are tested by the auto makers, with most, including Ford, strictly adhering to EPA protocol, while some OEMs blend the federal test procedures with real-world testing.

The controversy is occurring despite minor modifications the EPA has made to its tests beginning with the ’08 model-year, which include vehicle-specific data to better reflect the affect of real-world driving conditions on fuel economy.

The new fuel-economy estimates reflect conditions such as road grade, high speed/rapid acceleration, use of air conditioning, load and the effects of different fuel blends.

EPA lab uses dynamometer to measure vehicle fuel economy. “Under (the) EPA’s new methods, the new fuel-economy estimates for most vehicles will be lower,” the agency says in a regulatory announcement. “This is not because auto makers have designed the same vehicles to be less fuel efficient, it is because our new test methods take into account factors that have been missing or not accounted for in the current tests.”

Even with the modifications, some critics argue EPA tests are outdated and ill-suited to accurately gauge the fuel-saving benefits of new technologies such as hybrids.

“The tests they use were defined in the 1970s and no longer reflect how people drive in the real world,” Don Anair, research director-Clean Vehicles Program for the Union of Concerned Scientists, tells WardsAuto. New technologies, such as onboard fuel-economy displays, have heightened consumer awareness of the mileage they are achieving, he adds.

The EPA believes the modifications made to its test procedures will better reflect real-world fuel economy, including hybrids.

“In vehicles that achieve better fuel economy, such as gasoline-electric hybrids, new city estimates will be about 20% to 30% lower than today’s labels,” the agency says. “And new highway estimates will be 10% to 20% lower.”

Such a precipitous drop in advertised hybrid fuel-economy likely will affect auto makers’ sales campaigns. Ford already is adjusting its strategy, with marketing chief Jim Farley contending fuel-economy claims are falling on deaf ears.

“If 92 kids in a class of 100 are best-in-class, what does best-in-class mean anymore?” he tells WardsAuto. “Based on that confusing environment, some companies are going to be rewarded by introducing new ways for consumers to see real-world fuel economy.”