Gear Ratio Calculator: RPM, Torque, Speed
This gear ratio calculator covers two common jobs. In gear mode, enter the teeth on the driving and driven gears to get the gear ratio, the output speed in RPM and the output torque, so you can see the trade between speed and force. In vehicle mode, enter engine RPM, transmission gear ratio, rear axle ratio and tire diameter in inches to get road speed in miles per hour, or flip it around to see what RPM you will cruise at on the highway. It is useful for bikes, robotics, RC cars, trucks and anyone swapping tires or rear gears.
Gear Ratio Calculator
Gear ratio from teeth counts, plus vehicle speed from RPM, gear and axle ratio.
How to use the Gear Ratio Calculator
- Pick gears or vehicle speed mode.
- For gears, enter the driving and driven tooth counts plus input RPM and torque.
- For vehicles, enter RPM, transmission gear, axle ratio and tire diameter.
- Read the ratio, speed and torque results.
Gear formulas
Ratio = driven teeth ÷ driving teeth. Output RPM = input RPM ÷ ratio. Vehicle mph = RPM × tire diameter ÷ (gear × axle × 336.13).
Features
- Ratio from teeth counts
- Output RPM and torque
- Vehicle mph from RPM
- RPM at a chosen mph
- Tire diameter in inches
- Copy and print
Frequently asked questions
How do you calculate gear ratio?
Gear ratio = teeth on the driven gear ÷ teeth on the driving gear. A 12-tooth gear driving a 36-tooth gear gives 3:1.
What does a 3:1 gear ratio mean?
The input turns three times for every turn of the output. Output speed drops to one third and torque rises about three times, minus friction losses.
How do I calculate mph from RPM?
mph = RPM × tire diameter (in) ÷ (gear ratio × axle ratio × 336). At 2,000 RPM, 1:1 gear, 3.73 axle and 31 in tires, that is about 49.4 mph.
Do bigger tires change my speedometer?
Yes. Larger tires cover more ground per turn, so you go faster than the speedometer shows unless it is recalibrated.
What is a lower gear ratio?
In trucks, a numerically higher axle ratio such as 4.10 is called lower; it gives more pulling power and higher RPM at a given speed.