Gear ratio = number of teeth on driven gear ÷ number of teeth on driving gear
Gear ratio = diameter of driven gear ÷ diameter of driving gear
Identify the driving gear and the driven gear
Count the teeth on both gears
Divide the driven gear teeth by the driving gear teeth
Simplify the ratio if needed
For multiple gear stages, multiply each stage ratio together
To find output speed, divide input speed by the gear ratio
To find output torque, multiply input torque by the gear ratio