The formula
overall ratio = gear × final drive · km/h per 1,000 rpm = circumference × 60 ÷ overall ratio
Where it comes from
Between the crankshaft and the wheel there are two reductions: the gear you are in and the final drive, which is fixed. Multiplied together they give the overall ratio, which says how many turns the engine makes per turn of the wheel. With the circumference of the wheel you can then go from revolutions to kilometres per hour and back. It is the calculation that tells you, before you buy a car, what it will be turning at a hundred and twenty on the motorway.
How to work it out by hand
- Multiply the gear ratio by the final drive ratio
- Work out the circumference of the wheel: diameter times pi
- Divide the engine speed by the overall ratio: those are the wheel revolutions
- Multiply by the circumference and by sixty to get km/h
What is worth knowing
The wheel diameter is the whole assembly, not the rim: a 205/55 R16 is about 632 mm, from the rim plus twice the sidewall. The tyre comparison calculator works it out from the size code. Fitting a bigger wheel lengthens the gearing, so the engine turns less at a given speed, but it also makes the speedometer under-read and leaves every ratio longer than the manufacturer intended.