This is a small Java Script utility that converts tach speeds to actual speeds for a variety of tire and gear combinations. The speedometer gear recommendations are for a speedometer calibrated for 1000 revolutions per mile, and may not be applicable for your vehicle.
I hope you find this script useful.

Ring and Pinion
8.8 ratios - 3.08=40/13, 3.27=36/11, 3.31=43/13, 3.55=39/11, 3.73=41/11, 4.10=41/10
7.5 ratios - 3.08=37/12, 3.27=36/11, 3.45=38/11

Transmission Gear

Tire Size

Engine RPM