[sanddrag]

If you know the Output RPM and the motor RPM, you have your overall ratio. Speed is inversly proportional to torque. That means, when your speed goes down, the torque goes up. If the speed is cut in half, the torque is doubled.

Say you have a motor that is 5000 RPM with 2 N-m (newton-meters) of torque. It is put through a 12:1 reduction (or 1:12 depending on how you look at it) which makes the transmission output shaft speed (5000/12) 417 RPM. The torque on the output shaft will be 2*12 which is 24 N-m. Other common units of torque are pound-feet (called foot-pounds by some) represented by lb-ft and ounce-inches represented by oz-in.

I hope this helps.

If you have any more questions, just ask and I'll be back here to answer them.