Never was able to pass math in school, but I think this works- if one of you math gurus find an error- tell me. You only need to know the speed and gear ratio for the bike- Tire rolling legnth for one rev (mark the ground with you on the bike as you roll it). In my case a 24 inch wheel the distance is exactly 6 ft. One mile (5280) divided by one tire rev (6) = 880 X speed (in my case peak power flattened out at 25 MPH in second gear). 880 X 25 mph= 22,000 wheel revs per hour divided by 60 gives 366 revs of the wheel per minute. 366 X gear ratio 14.2 = 5206 RPM. This seems right for my HF79, which is a pushrod 4 stroke. Now if the bike was in 3rd gear, it would not pull max RPM. This test was to see how fast this motor spins over at peak power. Do I have the math right?