MPH from RPM Calculator
Calculate vehicle speed in MPH from engine RPM, gear ratio, axle ratio, and tire diameter.
How to use this tool
- Enter the engine RPM you want to evaluate.
- Enter the overall tire diameter in inches.
- Enter the transmission gear ratio for the selected gear and the axle ratio.
- The calculator returns vehicle speed in MPH.
Find your vehicle's true road speed from engine RPM and drivetrain specs.
⚠ This tool provides general estimates for education only and is not financial, tax or legal advice. Figures may not reflect your situation — verify with a qualified professional.
Formula
Speed (MPH) = RPM × Tire diameter (in) × π ÷ (Gear ratio × Axle ratio × 1056)
The constant 1056 converts inches per minute to miles per hour: 1 mile = 63,360 inches; 63,360 ÷ 60 min = 1056.
How it works
This calculator converts engine RPM to road speed by tracing the rotation through the gearbox and axle ratios to the rolling circumference of the tire. It assumes zero tyre slip and a perfectly round tyre at the stated inflated diameter. Tyre growth at high speed and actual loaded radius can cause small deviations from the calculated result.
Worked example
- RPM = 3000, Tire diameter = 26 in, Gear ratio = 1.0:1, Axle ratio = 3.73:1.
- Numerator: 3000 × 26 × π = 3000 × 81.681 = 245,044.
- Denominator: 1.0 × 3.73 × 1056 = 3938.88.
- Speed = 245,044 ÷ 3938.88 ≈ 62.21 MPH.
Speed = 62.21 MPH
Common mistakes to avoid
- Using the tire section width as diameter instead of overall tire diameter -- section width is the tread width, not the rolling diameter.
- Forgetting to include both the transmission gear ratio and the axle ratio; using only one understates total reduction and inflates the speed result.
- Not accounting for tire growth at high RPM or slip, which slightly increases effective rolling diameter and real-world speed.
Key terms
- Tire diameter
- The overall outside diameter of an inflated, unmounted tyre, usually measured in inches.
- Gear ratio
- The transmission's input-to-output speed ratio for the currently selected gear.
- Axle ratio
- The differential ring-and-pinion ratio that further reduces driveshaft speed before reaching the wheels.
- Rolling circumference
- The distance a tyre travels per revolution, equal to π × diameter; used to convert rotational speed to linear speed.
Frequently asked questions
- How do you calculate vehicle speed from RPM?
- Speed (MPH) = RPM × tire circumference (in) ÷ (gear ratio × axle ratio × 1,056). The constant 1,056 converts inches/minute to MPH (63,360 in/mile ÷ 60 min/hr).
- Why does gear ratio matter for speed?
- A lower (numerically higher) gear ratio like 3.73 gives more torque but less top speed than a ratio like 2.73 at the same RPM.