Gear ratio = number of teeth on driven gear ÷ number of teeth on driving gear
Gear ratio = driven gear diameter ÷ driving gear diameter
Gear ratio = output speed ÷ input speed
Gear ratio = input torque ÷ output torque
To calculate, divide the larger gear value by the smaller gear value
If using teeth counts, ratio = driven teeth / driver teeth
If using RPM, ratio = input RPM / output RPM
If using distance, ratio = larger circumference / smaller circumference
For compound gear trains, multiply each stage ratio together
For reduction, ratio > 1
For overdrive, ratio < 1
Output speed = input speed ÷ gear ratio
Output torque = input torque × gear ratio
To find required driven teeth, driven teeth = driver teeth × gear ratio
To find required driver teeth, driver teeth = driven teeth ÷ gear ratio