Gear ratio RPM calculator

Enter a gear ratio, final drive and tire size to find the theoretical engine RPM a road speed produces, or the speed a given RPM produces.

Formulaengine RPM = wheel RPM × gear ratio × final drive

How gearing links road speed and engine RPM

Wheel revolutions come from road speed and the theoretical circumference of the labeled tire size. Engine RPM multiplies wheel RPM by the selected gear ratio and the final drive, so the two ratios together decide the revs a given speed produces.

Operating conditions change the result

Tire growth and deflection, converter or clutch slip, drivetrain losses and calibration are not included. Use manufacturer data for permissible speeds and RPM.

A worked example

At 65 mph on a tire with a 78.45-inch circumference, the wheels turn about 875 rpm. In a direct 1.00 gear behind a 3.73 final drive that is 3,263 engine rpm. Swap to an overdrive sixth of 0.72 and the same road speed becomes 2,350 rpm.

That gap is what an overdrive gear is for, and it is also what makes a final-drive change feel like a different car. Going from 3.73 to 4.10 with everything else unchanged lifts cruising revs by 10%, sharpening acceleration and costing fuel economy at motorway speed.

Frequently asked questions

Where do I find my gear and final-drive ratios?

The owner’s manual or the workshop manual lists them per gear, and enthusiast references for a specific model and year are usually reliable. The final drive is sometimes stamped on the differential housing or encoded in the axle tag. Ratios often changed between model years and between engine or transmission options, so check the exact specification rather than the model name.

Why does my tachometer show something different?

Because a real drivetrain slips. An automatic with an unlocked torque converter runs several hundred rpm above the calculated figure, and even a manual loses a little to tire deflection and growth at speed. Once the converter locks up, the calculation usually lands within a couple of percent.

Can I use it to choose a final-drive ratio?

Yes, and it is one of the better uses for it. Fix the speed you cruise at, try candidate ratios and read the engine rpm each one gives. Set that against where your engine makes its torque and where it gets noisy, and the shortlist narrows quickly. What the arithmetic cannot tell you is whether the gearbox, driveshafts and axle will take it.

Does a taller tire change my gearing?

Effectively yes. A larger circumference lowers wheel rpm at a given speed, so engine revs drop and the car feels longer-geared: relaxed on the motorway, lazier away from a junction. Calculate the new circumference with the tire size calculator, then bring it back here.

Does this work for a motorcycle?

The gearbox and tire part does. For final drive, a chain-driven bike divides the rear sprocket teeth by the front, so work that ratio out on the sprocket ratio page and enter it as the final drive here. Shaft and belt drives publish a ratio directly.