How do you calculate the gear ratio on a bike?
Divide the number of teeth on the pedal gear by the number of
teeth on the wheel gear. If you have multiple gears to choose from,
you then have multiple ratios.
If you are talking about a motorcycle, the same principle
applies, but now you have more than one set of gears: the crank to
countershaft primary set, the selected gear set, and the mainshaft
to wheel set (if chain), or the two sets (mainshaft to shaftdrive
and shaftdrive to wheel, if shaft drive). You need to multiply the
ratios that are in series at any one moment of time to get the
overall gear ratio from crank to wheel.