Gear Ratio Calculator

Divide the number of teeth on the driven gear by the number on the driving gear. A 36-tooth driven gear meshing with a 12-tooth driver gives a 3.00 : 1 ratio, meaning the input turns three times for every one turn of the output.

A 3:1 reduction cuts output speed to one third of input speed and multiplies torque by three (minus friction losses). At 1500 input RPM you get 500 output RPM, and 100 N m of input torque becomes 300 N m at the output.

Wheel RPM equals engine RPM divided by the product of the transmission ratio and final drive (differential) ratio. Multiply wheel RPM by tire circumference (pi times diameter in inches), then divide by 1056 to get miles per hour. The 1056 constant comes from 5280 feet per mile times 12 inches per foot divided by 60 minutes per hour.

Total ratio is the product of each stage ratio. Two stages of 3:1 and 2:1 in series give an overall 6:1 reduction. The same multiplication applies to torque: input torque is multiplied by the total ratio, and output speed is divided by it.

Mechanical advantage equals the gear ratio itself (ignoring friction). A 4:1 reduction gives a mechanical advantage of 4, so 25 N m of input torque produces 100 N m at the output while the output spins four times slower than the input.