simple example is an e300 overvolted to 36v with stock gearing, 11-55 ratio
motor rpm is 2600 at 24v So 36/24=1.5 times 2600=3900rpm motor rpm
the Gearing is 5to 1 (5rpm at motor is 1 rpm at tire so 1/5th of 3900=780rpm(more math for complex gearing )
the tire is 10 inch so circumference is 10times Pi 3.14 =31.4 inches per rpm
31.4inches times 780 rpm =24492 inches per minute, divide by 12=2041fpm feet per minute
Divide 2041fpm into miles so (2041fpm /5280=.387miles per minute *
now converting to MPH=.387 times 60 minutes =23.19mph.. As per real world results this is darn close to the 22.5mph commonly expected from an overvolted e300 with stock gears.