Makers of the thriftiest cars and trucks on the road have less to brag about under new vehicle fuel-economy ratings the government says better reflect Americans’ real-world driving.

The U.S. Environmental Protection Agency recently released new vehicle fuel-economy estimates that, for the first time, account for current interstate highway speeds, heavier use of air conditioning and other modern-day driving habits.

The agency had not changed the way it evaluates cars and trucks for fuel economy since 1984.

Under the EPA’s new ratings system, city mileage for all ’07 U.S. light vehicles, on average, falls 12%, while average highway estimates drop 8%. Combined fuel economy for the current fleet is about 10% lower.

For some vehicles, particularly fuel-sipping powertrains, the new ratings system can mean as much as a 30% reduction in city fuel economy and a 25% cut in highway driving averages.

“The most efficient vehicles on the market are gas-electric hybrids,” an EPA spokesman says. “They’re more affected downward as a percentage than other vehicles. Smaller engines pay a higher penalty for accessory usage, such as air conditioning.”

For example, Toyota Motor Corp.’s Prius hybrid-electric vehicle saw its combined city/highway numbers fall from 55 mpg (4.2 L/100 km) to 46 mpg (5.1 L/100 km), or a 16% drop, according to the new measurements.

The impact is far less on trucks and SUVs. The ’07 Chevrolet Tahoe, for instance, when equipped with a V-8 5.3L engine, slipped from a combined rating of 18 mpg (13 L/100 km) to 16 mpg (14.7 L/100).

The new mileage ratings do not affect auto makers’ corporate average fuel economy totals.

scanderson@wardsauto.com