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.

What is the Automotive Gear Ratio & Speed Calculator?

Computes vehicle speed in mph or km/h at any engine RPM from the individual gear ratio, final drive ratio, and metric tire size. Builds a full RPM-versus-speed table for every gear, ideal for gear swaps, tire-size changes, and cruising RPM planning. It runs free in your browser on Gera Tools, with nothing uploaded.

Automotive Gear Ratio & Speed Calculator

Name: Automotive Gear Ratio & Speed Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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, check cruising RPM before a long highway trip, or understand what changed after a differential swap.

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 three-part size you enter (for example 225/45R17):

sidewall (mm) = section width × aspect ratio / 100
             = 225 × 45 / 100 = 101.25 mm
diameter (in) = rim diameter + 2 × sidewall / 25.4
             = 17 + 2 × (101.25 / 25.4) ≈ 24.97 in

Reading the results table

The tool generates one column per gear. At a given RPM, a lower gear ratio (numerically higher, like first) produces a lower speed; a higher gear ratio (numerically lower, like sixth) produces a higher speed. The final drive multiplies every gear uniformly — raising the final drive numerically drops speed per RPM across the board, while lowering it raises speed per RPM.

Practical examples

Tire upgrade check: you are moving from 225/45R17 to 235/45R17. The larger tire adds about 0.4 inches to the diameter, raising speed per RPM by roughly 1.6%. Your speedometer will read slightly slow, and your effective gearing becomes fractionally taller.

Cruising RPM on the highway: a 0.69 sixth gear with a 3.73 final drive and a 225/45R17 tire at 2,000 RPM works out to roughly 80 mph. If you swap to a 3.55 final drive, the same gear and RPM yields about 84 mph — your engine loafs more quietly on the highway.

Gear swap planning: fitting numerically lower (taller) gears raises top speed but hurts off-the-line acceleration and increases the RPM needed at low speeds. Numerically higher (shorter) gears sharpen acceleration and pull but raise highway RPM and fuel consumption.

Accuracy and limitations

The figures assume zero tire slip and the unloaded (free-rolling) tire diameter. Under load the tire deflects, shrinking the rolling radius slightly, so real speed is typically one to two percent lower than the calculated figure. Driveline losses are not modelled. Use the output for comparison and planning, then verify cruising RPM with an accurate tachometer on a known road speed.