Gear Ratio Calculator
Use the first mode for any pair of gears or sprockets. Use the second to see how axle ratio and tire size turn engine speed into road speed.
Result
- Torque multiplication
- 3 ×
- Effect
- Reduction: slower output with more torque
Formula
- Gear ratio = driven teeth ÷ driving teeth
- Output RPM = input RPM ÷ gear ratio
- Output torque = input torque × gear ratio (before losses)
- mph = RPM × tire diameter (in) ÷ (overall ratio × 336.13)
How the Gear Ratio Calculator works
A gear ratio compares the driven gear with the driving gear. A ratio above 1 slows the output and multiplies torque by the same factor. A ratio below 1 does the opposite and is called overdrive.
In a vehicle, the transmission ratio and the axle ratio multiply to give the overall ratio between the engine and the wheels. Road speed then depends on tire circumference. The constant 336.13 collects the conversions from inches per minute to miles per hour.
Worked example
A 12-tooth gear driving a 36-tooth gear gives 36 ÷ 12 = 3 : 1. At 1,500 RPM in, the output turns at 500 RPM with three times the torque. In a vehicle at 3,000 RPM in a 1 : 1 gear with a 3.73 axle and 28 in tires, speed is 3,000 × 28 ÷ (3.73 × 336.13) = 67.0 mph.
Frequently asked questions
Is a higher or lower gear ratio better?
Neither is better in general. A numerically higher axle ratio such as 4.10 gives stronger acceleration and towing. A lower one such as 3.08 gives lower engine speed and better fuel economy on the highway.
How do I calculate the ratio of a gear train?
Multiply the ratios of each stage. A 3 : 1 stage followed by a 4 : 1 stage gives 12 : 1 overall.
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Calculations run in your browser and nothing you enter is sent to a server. Results are provided for general information. Check critical figures independently before relying on them.