Tire Size Speedometer Calculator: Error Explained (2026)

Tyre (Tire) size and speedometer error at a glance

A car’s speedometer counts wheel turns, not distance. If you fit tyres with a larger overall diameter, every turn covers more road and the speedometer reads low, so you are driving faster than it shows. A smaller diameter makes it read high. Only the overall diameter matters. Width alone does not change the reading.

Why the overall diameter is what counts

The speedometer is calibrated at the factory for the tyre sizes the car was approved with. The car’s computer turns wheel or gearbox rotations into speed using a fixed figure for distance per turn. It cannot tell that you have fitted a different size, so a larger or smaller tyre throws off every speed and distance it shows by the same percentage.

The size on the sidewall mixes metric and imperial units, which is why working out the diameter by eye goes wrong. A typical marking has three parts:

• The first number is the section width in millimetres
• The second number is the aspect ratio: the sidewall height as a percentage of the width
• The last number is the rim diameter in inches

The overall diameter is the rim diameter plus two sidewall heights. Two sizes can share the same width and still differ in diameter if the aspect ratio or rim size changes. And a lower-profile tyre on a bigger rim can end up with almost the same diameter as the original. Fitters use this to keep the speedometer unchanged when they plus-size wheels. For a full breakdown of each part of the marking, see how to read tyre size numbers.

To compare two sizes before you buy, enter both into the tyre size calculator. It shows the difference in diameter and what the speedometer will display against your true speed, in km/h and mph.

How far off can a speedometer legally read?

Cars approved under UN Regulation No 39 (UNECE), used in the UK and across Europe, must have a speedometer that never shows less than the true speed. It may over-read within a margin. That margin is set as a percentage of true speed plus a fixed allowance. This is why most factory speedometers read slightly high and a GPS app often shows a lower figure.

The approval test is done on tyres the car is normally fitted with. A size change can use up that margin or push the speedometer below true speed. Below true speed is the risky case: you will be speeding while the dial says you are at the limit.

• UK: The Road Vehicles (Construction and Use) Regulations cover speedometers on cars. Going faster than the posted limit is an offence whatever the speedometer showed. A speedometer reading low after a tyre change is not a defence.
• US and Canada: Drivers are held to the posted limit, not to the dashboard. If you change the tire size, getting the speedometer to read correctly is your job.
• Australia: The Australian Design Rules for new vehicles include speedometer requirements. Police enforce the true speed measured by camera or radar.

If you are caught over the limit abroad, see speeding tickets abroad and the speeding fine calculator.

Is it illegal to drive without a speedometer?

In the UK, most cars must have a speedometer that works. A speedometer that has stopped working, or one that is far out after a modified tyre and wheel set-up, can be treated as a defect. In the US, Canada and Australia, the rules vary by state, province or territory. Everywhere, a car with no working speedometer leaves you no reliable way to keep to the limit. Get a failed speedometer repaired before any long trip.

What else moves the reading on the same tyre size

• Tread wear: A worn tyre has a slightly smaller diameter than a new one, so the speedometer reads a little higher as the tread wears down. New tyres bring it back.
• Pressure: An under-inflated tyre squashes more under load and rolls a slightly shorter distance per turn. Keep the pressure at the door-placard value. The tyre pressure calculator converts between bar and PSI.
• Mixed axles: Tyres with different wear on the front and rear axles can confuse cars that compare wheel speeds, and can confuse ABS and traction control too. Regular rotation keeps wear even; see the tyre rotation and alignment guide.
• Spare wheels: A temporary spare is often a different diameter. Treat the speedometer as approximate until the full-size tyre is back on.

What to do before and after a tyre size change

Before you buy:

1. Check the sizes listed in the handbook or on the door placard. Sizes listed there are already allowed for in the speedometer calibration.
2. If you want a size that is not listed, compare its overall diameter with the original in the tyre size calculator. Choose a size that keeps the diameter as close to the original as you can.
3. Keep the load index and speed rating at or above the specification. A new size can carry a lower load index than the original; see understanding tyre load index.

After fitting:

1. Drive at a steady speed on a straight, flat road and compare the speedometer with a GPS speed reading.
2. If the speedometer now reads below GPS, have it recalibrated. Many newer cars can have the tyre size changed in their software by a dealer or specialist. Until then, keep below the limit by the size of the error.
3. Tell your insurer about any non-standard wheel and tyre size, because it counts as a modification.
4. Remember that the odometer is off by the same percentage, and that affects service intervals and recorded mileage.

Sources

• UN Regulation No 39: speedometer and odometer equipment, including its installation (UNECE)
• Road Vehicles (Construction and Use) Regulations (legislation.gov.uk)
• Australian Design Rules: vehicle standards (Australian Government)