If you drive a hybrid, you have to tread lightly on the throttle and give up luxuries such as, oh, air conditioning, if you hope to get anywhere near the EPA test figures for fuel economy. The disparity should diminish starting with 2008 cars, when the EPA introduces tougher testing rules. Your mileage won't improve; the test mileage will go down.

The EPA estimates mileage figures will be 10 to 20 percent lower for city driving and 5 to 15 percent lower for highway driving, for traditional vehicles. For hybrids, it may be 20 to 30 percent lower for city driving and 5 to 15 percent lower for highway driving. Hybrids get the best gas mileage under mild acceleration and with air conditioning off -- and those kinds of test conditions will go by the wayside.

The EPA says the revisions, the first since 1985, are meant to reflect changed driving conditions. But they also are happening because some EPA tests were designed in the 1970s, when test gear was wimpier; the dynomometers (big rollers under the drive wheels in the labs) couldn't handle real-world acceleration, and now they can.