question: are you using gears?, if not then its 1:1, that means motor rev per minute times tyre(wheel) circumference gives meters done per minute * 60 / 1000 = kmh, you may have a ratio motor to drive shaft if the motor is not directly driving the wheels, my mitsubishi express has a fix ratio of 4.625 (motor to gears) and then the ratio depending of which gear i'm driving, result if i have 2500rpm & 5th gear in my van moves at 100kmh (144v 24kw dc motor, max 5000rpm)