Joel,

Whats always worked for me is:

                     RPM X Tire diameter       (say 25" tall for example)
MPH =          ~~~~~~~~~~~~~~~~
                    Final drive ratio X  336
   (final drive= ring and pinion X gear ratio)
                          4.86                .82

So,   at 3500 rpm with 25" tall tires and a final drive of 3.985 you
are going 65.35 mph.

Todd Hill
VolksWerks Transaxles
Olympia, WA