What formula converts RPM to road speed?

Wheel RPM equals engine RPM divided by the product of the gear ratio and final drive ratio. Multiplying wheel RPM by the tire circumference and converting units gives road speed. The tool uses 63360 inches per mile and pi times tire diameter for circumference.

How is tire diameter calculated from a size like 225/45R17?

Sidewall height in millimetres is section width times aspect ratio divided by 100. Overall diameter in inches is the rim diameter plus two sidewalls converted from millimetres at 25.4 mm per inch. So 225/45R17 is about 24.97 inches tall.

Why is my real speed slightly lower than the tool shows?

The calculation assumes no tire slip and uses the unloaded tire diameter. Under load the tire deflects and the rolling diameter shrinks slightly, and there is minor driveline slip, so actual speed is typically one to two percent below the calculated figure.

How does changing the final drive ratio affect speed?

A higher numerical final drive (say 4.10 instead of 3.73) multiplies torque but lowers speed per RPM and raises cruising RPM. A lower numerical ratio does the opposite, giving taller gearing, lower highway RPM, and better fuel economy at the cost of acceleration.

Can I use this to check cruising RPM for a tire upgrade?

Yes. Enter your top gear and final drive, set the RPM to your typical cruise figure, then compare the speed with your old and new tire sizes. A taller tire raises speed per RPM, which lowers the RPM needed for a given highway speed.

Automotive Gear Ratio & Speed Calculator

Gearing, final drive, and tire diameter together determine how fast a vehicle travels at any engine speed. This calculator turns those three inputs into a clear table of road speed in every gear, so you can plan a gear swap, evaluate a tire upgrade, or find your true cruising RPM.

How it works

Road speed is derived from the rotational chain between the crankshaft and the contact patch:

wheel RPM   = engine RPM / (gear ratio × final drive)
circumference (in) = π × tire diameter (in)
speed (mph) = wheel RPM × circumference × 60 / 63360
speed (km/h) = mph × 1.609344

Tire diameter is built from the metric size you enter:

sidewall (mm) = section width × aspect ratio / 100
diameter (in) = rim (in) + 2 × sidewall (mm) / 25.4

Example and notes

A 225/45R17 tire is about 24.97 inches tall. In a 0.69 sixth gear with a 3.73 final drive at 2000 RPM, that works out to roughly 80 mph. The figures assume zero slip and the unloaded tire diameter, so plan for real speed about one to two percent lower. Taller (lower-numerical) gearing or a larger tire both raise speed per RPM, which lowers highway RPM and usually improves fuel economy while softening acceleration.