Here's some formulas I've found to be quite useful...

FINAL DRIVE RATIO =                                                                                                      R&P gear ratio X desired transmission gear ratio
Example - 4.37 R&P X .89 4th gear ratio = 3.889 Final Drive

MPH =                                                                                                                             RPM X Tire Diameter In Inches, Divided By, Drive Ratio X 336
Example - 3500 rpm X 28 inches = "98000" divided by 3.88 final drive(4.37 r&p X .89 4th gear) X 336 = "1303.68" 98000 / 1303.68 = 75 mph

RPM =                                                                                                                             MPH X DRIVE RATIO X 336, Divided By, TIRE DIAMETER
Example - 65 X 3.88 X 336 = 84739.2 / 28 = 3026 rpms

Engine Size In Cubic Centimeters *cc's*=                                                                            BORE "mm" X BORE "mm" X STROKE "mm" X .0031416 = 1915cc

SINGLE CYLINDER DISPLACEMENT=                                                                             BORE "mm" X BORE "mm" X STROKE "mm" X .0031416, divided by 4
Example - 94mm X 94mm X 69mm X .0031416 = 1915 / 4 = 478.75

 

Have Fun, Fun, Fun... ManxManiac