Drivetrain and performance

Gear Ratio, RPM & Speed Calculator

Estimate road speed from engine RPM or calculate RPM at a target speed. Enter tire diameter, transmission gear and final drive ratio to model the complete drivetrain.

Calculations run locally without sending your input values

Drivetrain inputs

Tire, transmission and axle

Tire diameterinTransmission gear ratio1.00 in direct drive:1Final drive / axle ratio:1Engine speedRPMTarget road speedmph

How to calculate speed from gear ratio

Vehicle speed depends on engine RPM, tire diameter, the selected transmission gear and the final drive ratio. The transmission and axle ratios are multiplied to produce the overall ratio. A numerically higher overall ratio increases engine RPM at a given road speed.

mph = RPM × tire diameter ÷ (overall ratio × 336)

The 336 constant is a practical approximation that combines inches, miles and minutes. Real speed can vary with tire growth, tread wear, converter slip and drivetrain losses.

Transmission ratio vs. final drive ratio

A transmission ratio describes the selected gear. A 1.00:1 gear is direct drive, while a value below 1.00 is overdrive. The final drive or axle ratio is the reduction in the differential. Multiply both values before calculating wheel speed. For example, 0.75 × 4.10 produces a 3.075 overall ratio.

How tire diameter changes RPM

A taller tire travels farther per revolution, lowering RPM at the same road speed. A shorter tire raises RPM. Use the loaded tire diameter when available because the advertised diameter can be slightly larger than the effective rolling diameter on the vehicle.

RPM to speed table

Same tire and ratios at common engine speeds.

RPM	mph	km/h
1,000	21.54	34.67
2,000	43.09	69.34
3,000	64.63	104.01
4,000	86.17	138.68
5,000	107.72	173.35

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Gear ratio calculator FAQ

What gear ratio should I enter for overdrive?

Enter the ratio of the selected transmission gear, such as 0.75:1, plus the separate axle ratio. Do not multiply them before entering the values.

Why does actual speed differ from the calculator?

Real tire diameter, tread wear, tire growth, torque-converter slip and measurement error can change the result. Treat the output as an engineering estimate.

Does a higher axle ratio increase RPM?

Yes. A numerically higher axle ratio raises engine RPM at the same road speed and generally multiplies torque more strongly.