What is the formula for RPM to speed?

Speed (mph) = (RPM × Tire Circumference in inches) ÷ (Gear Ratio × 1056). The 1056 constant converts inches-per-minute to miles-per-hour (63360 inches/mile ÷ 60 min/hour = 1056). Tire circumference = π × tire diameter in inches.

What gear ratio should I enter — transmission or axle?

Enter the combined (total) gear ratio: multiply the selected transmission gear ratio by the rear axle (differential) ratio. For example, if you are in 3rd gear at 2.85 and your axle ratio is 3.73, enter 2.85 × 3.73 = 10.63.

How do I find my tire diameter from a P-metric size?

Use the tire code fields on this calculator — enter a size like 225/45R17 and it computes the diameter automatically. The formula is: diameter = rim diameter × 25.4 + 2 × (section width × aspect ratio / 100), then convert mm to inches by dividing by 25.4.

Why does changing tire size affect my speedometer?

Your speedometer counts wheel rotations per unit time. A larger diameter tire travels further per rotation, so the same RPM produces a higher actual speed. The speedometer underreads if tires are bigger than stock.

What is a typical axle (final drive) ratio?

Common rear axle ratios range from 2.73 (fuel-economy focused) to 4.56 or higher (towing/off-road). Passenger cars often use 3.31–3.73. Check your owner's manual, door sticker, or axle tag for your specific ratio.

Can I use this for front-wheel-drive vehicles?

Yes. FWD vehicles use the same formula — enter the total gear reduction from crank to wheel (transmission gear × final drive ratio inside the transaxle). The math is identical regardless of which wheels are driven.

RPM to Vehicle Speed Calculator

Name: RPM to Vehicle Speed Calculator
Availability: InStock
Author: Nham Vu

Enter your engine RPM, gear ratio, and tire size to instantly calculate your vehicle speed in mph and km/h.

Drivetrain Inputs

Engine RPM

Crankshaft rotations per minute.

Total Gear Ratio

Transmission gear ratio × axle ratio (e.g. 2.85 × 3.73 = 10.63).

Tire Size

Paste the tire code to auto-fill the dimensions.

Width (mm)

Aspect %

Rim (in)

Enter RPM, gear ratio, and tire size, then click Calculate Speed.

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

Related tools

Tire Size Calculator Speedometer Error Calculator Gear Ratio Calculator

Last updated: 2026-05-29 · Reviewed by Nham Vu