The Environmental Protection Agency has revised the formula for calculating official fuel mileage ratings to try and make them more representative of actual performance, the New York Times reports. The changes are intended to account for how car owners actually use their vehicles, as this effects fuel economy. The result is lower estimated miles-per-gallon for 2008 vehicles.

he system was last adjusted in 1984, when the E.P.A. cut the city values by 10 percent and the highway values by 22 percent. This one cuts the estimate of highway mileage another 8 percent, on average, and city mileage up to another 25 percent beyond the 1984 cut. Generally, the new formula narrows the gap between small cars and bigger vehicles, with small-car fuel economy estimates falling by more than the estimates for sport utility vehicles and other larger vehicles.

Hybrid vehicles will take some of the biggest hits, according to the agency, because the new standards include use of heaters and air-conditioners. Most gas-electric hybrids shut down their engines at low speeds, and run on electric batteries, but if the vehicle occupants have turned on the air-conditioner or heater, the engine may not shut down.

Despite what the EPA's best efforts, it remains the case that "actual mileage may vary."