Speedometer error calculator

What the band means

The regulation is deliberately one-sided. The lower bound is zero, because a car that under-reads could put a driver over a limit while the dashboard says otherwise. The upper bound is generous because the same band has to absorb tyre wear, pressure, load and every approved tyre size a model is sold with — so manufacturers bias the reading upward and keep clear of the zero line.

Most cars use a few per cent of the allowance rather than all of it. If your dashboard sits near the top of the band, the usual cause is tyres with a different rolling circumference from the ones the car was approved on. The full explanation is here, and the conversion pages show the same band for individual speeds.

Common questions

How much is a car speedometer allowed to over-read?

Under UN Regulation No 39 the indicated speed may exceed true speed by up to 10% plus 4 km/h, and may never fall below true speed. At a true 100 km/h that permits a display of up to 114 km/h.

Can a speedometer legally read low?

No. The regulation sets the lower bound at zero error: the speed indicated shall not be less than the true speed of the vehicle. A speedometer reading below true speed is out of specification.

Does this calculator prove my speedometer is faulty?

It shows whether a reading falls inside the legal band. A dashboard reading above the band suggests a problem, most often non-standard tyres, but a consumer GPS reading is not a calibrated measurement and is not evidence on its own.