(Shared from eHow.com)

The gear box ratio is the difference between the RPM produced by the motor and the RPM measured after the gearbox. A gear box can either increase or decrease the RPM of a motor. A gear box ratio is noted A:B where A>B in a speed reducer and A<B in a multiplier.

Instructions

1.       Write down the rated RPM of the motor. RPM_motor = 10,000RPM in our example.

2.       Turn the motor on and use a tachometer to measure the RPM after the gearbox. Write that number down. RPM_gearbox = 2500RPM in our example.

3.       Divide the RPM_motor by RPM_gearbox and write down that number. In our example, 10000/2500 = 4 so our ratio is 4:1.

4.       Simplify the fraction RPM_motor/RPM_gearbox instead of calculating the ratio.

Tips & Warnings

The RPM value given by the motor manufacturer always come with a +/- value. That value tells you that the RPM speed varies by that percentage. For example, a 1000RPM motor with a +/- .5% value actually turns between 995 and 1005 RPM. Because of that fact, the RPM value after the gearbox may vary slightly and give you an odd number, for example 4.089:1. If it is the case, round that number to get a clean ratio of 4:1.