Measure the radius from the ground to the center of the rim. Double that number. Divide 20,178 by that number. That is your tire RPM @ 60 MPH. Mulitiply by your gear ratio and that is your RPM @ 60mph. (no slip) example. Tire =14" x 2 =28. 20178/28 = 720. 720 x 3.50 gear =2520 @60 mph