This is a very simple calculator which takes dimensions of the model wheel and the real machine top speed to calculate the motor rpm  you would need to achieve the scaled speed with your 1:32 scale model.

It works by using the wheel diameter to calculate the circumference of the wheel which is the distance travelled in one revolution of the wheel. The top speed on the real 1:1 scale machine is divided by 32 to give the 1:32 scale equivalent of the the speed. This value is then divided by 60 to give km/min and multiplied by 1000000 to give mm/min. Dividing this value by the circumference of the wheel gives us the rpm of the wheel required to achieve that speed which is usually the rpm of the gear motor because the axle is usually the output of the gear motor.