This program will calculate the speed of a vehicle for every gear. If you have a trans-axle use 1 for the rear end ratio.
For those interested this is accomplished by dividing the RPM by the final gear ratio then multiplying the circumference of the tire.
Number of transmission gears
Rear end gear ratio
This calculator will assist you in determining the actual size of your tire. Currently very limited and only usable for passenger car tires.