How the conversion works
The speedometer counts wheel revolutions. If the fitted tyre has a larger circumference than the original, each revolution covers more ground, so the car is travelling faster than the dial suggests. The ratio is fitted diameter divided by original diameter.
Actual speed = indicated speed x fitted diameter / original diameter. The error is that ratio minus one, shown as a percentage, and the difference at the current reading is the gap in mph.
Worked example
A car was calibrated for a 632 millimetre diameter and now runs a 652 millimetre tyre. At an indicated 70 mph the ratio is 1.0316, so the actual speed is 72.2 mph, an error of 3.2 per cent and a difference of 2.2 mph. The same car shows 30.9 mph at an indicated 30 and 51.6 mph at an indicated 50.
These figures are illustrative. Measure the diameters, or take them from the tyre size codes, for your own vehicle.
Why a speedometer may already be out
Speedometers are allowed to over-read. The UNECE regulation that governs them, R39, permits an indicated speed between the true speed and the true speed plus 10 per cent, plus a small fixed allowance, and does not permit under-reading. Many cars therefore show a few per cent more than the real speed on standard tyres.
That built-in over-read partly offsets the error from a slightly larger tyre. A change that makes the dial read 2 per cent low against the original calibration may simply cancel an over-read the car already had.
Where the diameters come from
The original diameter is the size the vehicle was built and calibrated with. The fitted diameter is the tyre on the car now. Both can be calculated from the size code using the tyre size calculator, or measured across the wheel with a straight edge.
Wear matters: a tyre worn from 8 millimetres to 2 millimetres of tread is roughly 12 millimetres smaller in diameter, which is about 2 per cent on a typical car tyre. Inflation changes the rolling radius too.
Limits of this calculator
It models the diameter ratio only. It does not account for tyre growth at speed, slip, wear, load or the calibration curve of a particular instrument, and it does not adjust an odometer or a digital speed limit display.
A speedometer that reads incorrectly can be a legal problem and an MOT matter. Have the vehicle checked by a specialist rather than relying on an arithmetic estimate.
To turn two size codes into diameters, use the UK tyre size calculator.
For US spelling and tire terminology, use the US speedometer calculator.
Inputs for the speedometer calculator
Speed shown on the speedometer
Enter the reading you see on the dial at the moment you want to check.
Original tyre diameter
The overall diameter of the size the speedometer was calibrated for, in millimetres.
Fitted tyre diameter
The overall diameter of the tyre now fitted, measured or calculated from its size code.
CALCULATOR QUESTIONS
Speedometer calculator UK questions
Is a speedometer allowed to under-read?
No. UK and UNECE rules require the indicated speed to be at least the true speed, so a dial may over-read but must not under-read.
Does a bigger tyre make the speedometer read high or low?
Low. Each wheel revolution covers more ground, so the real speed is higher than the number on the dial.
Will this fix my odometer as well?
It shows the same percentage applies to distance, but it does not recalculate or adjust the odometer reading.
How accurate are the diameters I enter?
The result is only as good as the figures. Nominal size codes ignore wear, inflation and manufacturing tolerance.