Engine RPM to Road Speed Calculator

Method and units

Wheel RPM = engine RPM ÷ (gear ratio × final-drive ratio). Speed in mph = wheel RPM × π × tire diameter (in) × 60 ÷ 63,360. The result is nominal kinematic speed, not an instrument correction.

Worked example

At 3,000 RPM, a 26-inch tire, 0.80 gear and 3.73 final drive give 1,005.4 wheel RPM and about 77.8 mph.

Supported inputs and interpretation

RPM 1–20,000; tire diameter 5–60 in; each ratio 0.01–20. Results are rounded for display only; internal calculations retain full JavaScript precision. Entered measurements and assumptions determine the answer.

References and limitations

The equations here use the stated unit conversions and geometric or accounting relationships; no empirical vehicle limits or manufacturer presets are supplied.

Assumes no clutch or torque-converter slip and uses nominal tire diameter; loaded rolling radius and actual circumference differ. Not a speedometer calibration or proof of safe road speed. Verify actual ratios, tire data and vehicle limits.

Related calculators

• Tire Size Speedometer Calculator
• Gps Speedometer Calibration

This planner performs the stated arithmetic; it is not a vehicle inspection, engineering approval, manufacturer's recommendation or professional review.