diameter, either go to one of the websites that offers the formulas to assist you in doing so, or go to my website ([ www.leadergears.com ]) where the formulas have been sitting for use for just that reason. Thanks Shawn for making it a plug in formula and much simpler, but for those who want an exact MPH vs. RPM in their application, do a little math with the formulas and get the numbers dead on. Take care, Mark P.S. Are you still able to keep up with traffic on the freeway? Talk to you this weekend.