Speedometer error estimate

Estimate road speed from indicated speed when only the theoretical tire diameter changes.

Formulaestimated speed = indicated speed × new diameter ÷ reference diameter

Distance per revolution changes

If the assumed reference calibration is exact, a larger rolling circumference travels farther per wheel revolution. The formula scales indicated speed by the diameter ratio.

Real rolling radius varies

Wear, pressure, load, vehicle calibration and legal tolerances can change the relationship. Use proper instruments and manufacturer procedures for calibration.

Your speedometer was already optimistic

Before any tire change, most speedometers read slightly high by design. UNECE Regulation 39, which applies across Europe and many other markets, requires that the indicated speed is never below the true speed and never more than 10% plus 4 km/h above it. Manufacturers aim safely inside that window, so a genuine 100 km/h commonly shows as 103 to 106.

That built-in bias is why fitting a taller tire can make a speedometer look accurate for the first time. The error has not gone away; two errors are cancelling. Anything that matters, a speed limiter, a track session, a legal question, needs a GPS reference rather than either number.

Frequently asked questions

Do bigger tires make the speedometer read high or low?

Low. A larger diameter covers more ground per revolution, so the car is travelling faster than the dial says. Smaller tires do the reverse and make it read high. The size of the error tracks the diameter change almost exactly: 3% taller means about 3% understated.

Does the odometer drift as well?

Yes, by the same proportion and in the same direction, because both are counted from wheel rotation. Taller tires under-record distance, which quietly stretches service intervals, understates the miles on a lease and flatters the trip computer’s fuel consumption figure.

Can the speedometer be recalibrated for a new tire size?

Often, but how depends on the vehicle. Some allow the tire size to be set in the instrument cluster or by the dealer with a diagnostic tool, some need a third-party recalibration module, and older cable-driven speedometers need a different drive gear. On many cars the same wheel-speed signal feeds ABS and stability control, which is why this is a job for the manufacturer’s procedure rather than a workaround.

How do I check my real speed?

A GPS speed readout on a phone or a satnav is the practical reference, and it is independent of your wheels entirely. Compare it on a level road at a steady speed, not while accelerating, and take a couple of readings. Roadside speed-display signs are a rough second opinion.

Does this account for worn tires?

No. It compares two labelled sizes as new. A tire worn from full tread to the wear bars loses a few millimetres of radius, so a set of well-used tires reads marginally slower than the same set when new. That drift is small compared with a size change, but it goes in the opposite direction to fitting something taller.