How the conversion works
The speedometer counts wheel revolutions. If the fitted tire has a larger circumference than the original, each revolution covers more ground, so the vehicle 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 vehicle was calibrated for a 632 millimetre diameter and now runs a 652 millimetre tire. 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 vehicle 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 tire size codes, for your own vehicle.
Why a speedometer may already be out
Speedometers are permitted to read high. Federal rules and the UNECE regulation that most manufacturers design to allow an indicated speed at or above the true speed, within a tolerance, and do not permit under-reading. Many vehicles therefore show a few per cent more than the real speed on standard tires.
That built-in over-read partly offsets the error from a slightly larger tire. A change that makes the dial read 2 per cent low against the original calibration may simply cancel an over-read the vehicle 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 tire on the vehicle now. Both can be calculated from the size code using the tire size calculator, or measured across the wheel with a straight edge.
Wear matters: a tire 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 tire. Inflation changes the rolling radius too.
Limits of this calculator
It models the diameter ratio only. It does not account for tire 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 display.
If the speedometer reads incorrectly, have the vehicle checked by a specialist. An arithmetic estimate is not a calibration.
To turn two size codes into diameters, use the US tire size calculator.
For UK spelling and tyre terminology, use the UK speedometer calculator.
Inputs for the US speedometer calculator
Speed shown on the speedometer
Enter the reading you see on the dial at the moment you want to check.
Original tire diameter
The overall diameter of the size the speedometer was calibrated for, in millimetres.
Fitted tire diameter
The overall diameter of the tire now fitted, measured or calculated from its size code.
CALCULATOR QUESTIONS
Speedometer calculator US questions
Is a speedometer allowed to under-read?
No. The applicable rules require the indicated speed to be at least the true speed, so a dial may read high but must not read low.
Does a bigger tire 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 that 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.