Jan Heine in Bicycle Quarterly has a method that could work for you. Find a long hill and coast down it without pedaling with two bikes and time the run. I would think one steep enough to go over 20 mph would handle the air drag issue fairly well. Give each bike the same pedal stroke to get going. Mr Heine uses this more to test tires, I believe,than bikes but it should work for both. Be interesting to know if Schwalbe Marathon Pluses,for example, compare to Primo Comets or the cheap 100 lb Kenda Kwests. of course wind or the lack of thereof would be a factor. i was thinking that to be fair you would have to have similar types of tires, handlebars (for wind resistance etc ) but maybe not. At the end of the day I would want to know how the bikes as I had them set up would compare.