Here's a colorized Road Load Horsepower graph of a 'typical' car, circa 1996, from Honda R&D, presented in a JSAE Paper for the 1996 Honda Dream, tandem solar racer.
You'll notice how the load varies with velocity.
We'd want to 'fix' this sort of thing when doing any comparisons.
http://i1271.photobucket.com/albums/...titled7_25.jpg