The horsepower ratings are average for a sample of vehicles tested. Done on a chassis dyno usually, which gives the actual power applied to the road, usually less that advertised based on drive train losses and the like. But, some engines are rated directly at output from the crank with a specified load. Also the horsepower is specified at a rpm number. So, unless you drive at 4500-5500 rpm or whatever, you are usually operating at less than the advertised power rating. Actually, who cares, if you like how it feels in the seat of your pants then make believe you've got 500 hp.
[Edited by jeffsr on 07-16-2001 at 02:30 PM]