Often cyclists ask us how they can tell the true size of the gear on a bicycle. The size of a gear corresponds with how far the bicycle will roll with one complete revolution of the pedal. Keep in mind that the size of wheel and tire can affect the effective size as well. These gear inches correspond to a 700cc wheel. The exact equation for figuring gear inches:
Diameter of drive wheel (rear) in inches × number of teeth in front chainring / number of teeth in rear sprocket.
gi = dwd x fct/rct
gi = gear inches
dwd = drive wheel diameter
fct = number of front chainring teeth
rct = number of rear sprocket teeth
Bicycle Gear Chart