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Gear ratio speed calculator

Work out how fast your car goes in each gear, what the engine is turning at motorway speed and how a different tyre size changes your speedometer reading. Everything updates as you type.

Published: · Last reviewed:

Inputs

Tyre size
Driveline
Gear ratios

Example values, not taken from any specific vehicle. Enter your own from the owner’s manual or gearbox specs.

Compare with another tyre (optional)

Results

How to use it

  1. Tyre size: copy the three numbers from the sidewall. On a tyre marked 205/55 R16, width is 205, aspect is 55 and rim is 16.
  2. Final drive and gear ratios: these come from your owner’s manual, the gearbox specification or a technical data sheet for your exact engine and gearbox. The pre-filled values are only an example.
  3. Rev limit: the rpm at which the engine cuts fuel. The tool uses it for the top speed in each gear.
  4. Optionally enter a second tyre size to see the change in diameter and how far off your speedometer will be.

The formulas

Tyre diameter (mm) = rim (in) × 25.4 + 2 × width × aspect / 100

Circumference (m) = π × diameter / 1000

Road speed (km/h) = rpm × circumference × 60 / (gear ratio × final drive × 1000)

Speedometer difference (%) = (new diameter / original diameter − 1) × 100

The rpm after an upshift is the rev limit multiplied by the next gear ratio and divided by the current one, assuming you shift exactly at the limiter.

Worked example

A 205/55 R16 tyre has a sidewall of 205 × 0.55 = 112.75 mm and a rim of 16 × 25.4 = 406.4 mm, so its diameter is 406.4 + 2 × 112.75 = 631.9 mm and its circumference is about 1.985 m. In a gear of 1.00 with a final drive of 4.00, at 3,000 rpm the car covers 3,000 × 1.985 × 60 / 4,000 ≈ 89.3 km/h.

Fitting a 225/45 R17 instead gives 634.3 mm, 0.38 % larger. When the speedometer reads 100 km/h you would actually be doing about 100.4 km/h.

Limitations

Results use the unloaded tyre geometry implied by its size marking. Real figures differ a little because:

  • under load the tyre deflects, so the effective rolling radius is somewhat smaller than the nominal one;
  • actual dimensions vary by manufacturer, model, inflation pressure and tread wear;
  • wheel slip, aerodynamic drag and available power are not modelled. The top speed shown for the highest gear is the rev-limited figure, not what the car can actually reach.

Factory speedometers are usually set to over-read slightly by design, so a percentage change here is not the same as your speedometer’s total error. Check legal and insurance requirements before changing tyre sizes.

Sources

Every formula on this page is covered by automated tests with hand-calculated reference values. Found an error? See the about page.

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