Based on the new test, the combined city-highway mileage for the 2007 Prius dropped from 55 miles per gallon to 46, a 16% decline. The Honda Civic hybrid, also a 2007 model, showed a similar decline.

Drivers have long complained that the EPA's mileage test, developed in the mid-'70s, didn't take into account the way people actually drive. For instance, the test didn't measure the effects of stop-and-go driving and lead-footing -- two common Southern California pastimes -- on fuel economy.

In addition, the national speed limit was 55 mph when the old test was developed. Today, it's as high as 80 mph in some places.

In 2005, Consumer Reports tested 303 cars and trucks and found that about 90% got lower mileage than the EPA's estimates. The mileage for city driving averaged about 30% below the government's figures.

(The fact that the new, lower estimates won't be used to calculate the car companies' average fuel economy rating -- known as CAFE -- also makes the pill easier to swallow.)

And the EPA is careful to note that it still isn't promising perfection.

"It's a sticker, not a crystal ball," Millett said. "This is a yardstick to compare one vehicle to another.