RPM to Vehicle Speed Calculator
Enter your engine RPM, gear ratio, and tire size to instantly calculate your vehicle speed in mph and km/h.
Drivetrain Inputs
Crankshaft rotations per minute.
Transmission gear ratio × axle ratio (e.g. 2.85 × 3.73 = 10.63).
Tire Size
Paste the tire code to auto-fill the dimensions.
Overall outside diameter in inches.
Enter RPM, gear ratio, and tire size, then click Calculate Speed.
Miles per Hour
—
mph
Kilometers per Hour
—
km/h
Calculation Breakdown
Engine RPM
—
Total Gear Ratio
—
Tire Diameter
—
Tire Circumference
—
Wheel RPM (after gearbox)
—
Speed vs RPM (current gear & tires)
Summary
Enter your engine RPM, gear ratio, and tire size to instantly calculate your vehicle speed in mph and km/h.
How it works
- Enter the engine RPM — the current or target rotational speed of the crankshaft.
- Enter the total gear ratio — multiply the transmission gear ratio by the axle/differential ratio (e.g. 2.85 transmission × 3.73 axle = 10.63 total).
- Choose your tire input method: paste a P-metric code like 225/45R17, or enter the tire's overall diameter in inches.
- Click Calculate to see vehicle speed in both mph and km/h.
- Adjust any value and click Calculate again to compare different scenarios.
Use cases
- Verify that a given RPM at highway cruising matches your expected speed.
- Plan gear ratios for a custom build or transmission swap.
- Evaluate how larger or smaller tires affect cruising RPM at a target speed.
- Find the RPM needed to reach a specific speed in any gear.
- Compare stock vs aftermarket axle ratios side by side.
- Determine optimal shift points for fuel economy or performance.
- Check whether an engine swap will keep RPM in the powerband at typical road speeds.
- Diagnose speedometer inaccuracies caused by a tire size change.
Frequently Asked Questions
Last updated: 2026-07-01 ·
Reviewed by Nham Vu