Skip to main content
Tool Belt Calc

Automotive

Speedometer Error Calculator

True speed and error after fitting a different tire size.

Your measurementsEvery assumption editable01
Actual speedCalculated live02

Result

60.1mph actual

Reading 60 — travelling 60.1

Original diameter
26.63 in
Fitted diameter
26.68 in
Ratio
× 1.0019
Indicated speed
60 mph
Actual speed
60.1 mph
Errorreading low — you are going faster than it says
+0.1 mph

A larger tyre travels further per revolution, so the speedometer under-reads and you are going faster than it shows. A smaller tyre does the reverse. The odometer is wrong by exactly the same proportion, which quietly affects service intervals and resale mileage.

Many speedometers are calibrated to read slightly high from the factory, because regulations in most markets allow reading over the true speed but never under. That factory bias sits on top of any error introduced here.

Some vehicles can be recalibrated for a different rolling diameter through the body control module or a dedicated tuner. Whether yours can is a model-specific question.

Enter your original and fitted tyre sizes and an indicated speed. A larger tyre makes the speedometer under-read — you are going faster than it says — and the odometer is wrong by exactly the same proportion.

Why use this tool?What it does differently03

Why use this tool?

Direction stated

Says whether you are travelling faster or slower than indicated, not just the size of the error.

Any indicated speed

The error scales linearly, so check it at motorway speed rather than at 30.

Odometer flagged

Mileage is wrong by the same proportion, which quietly shifts service intervals and recorded mileage.

Factory bias explained

Most speedometers read slightly high from new, and that sits on top of any error here.

How this worksThe method04

How this speedometer error calculator works

The ratio of fitted diameter to original diameter is the correction factor. Multiply the indicated speed by it and you have true speed — a five per cent larger tyre means you are travelling five per cent faster than the dial shows.

The same factor applies to the odometer. A car fitted with tyres five per cent taller accumulates recorded mileage five per cent slower than it actually travels, which shifts every distance-based service interval.

Many speedometers are calibrated to read slightly high from the factory, because regulations in most markets permit reading over true speed but never under. That deliberate bias adds to whatever a tyre change introduces.

How to use itStep by step05

How to use it

  1. Step 1: Enter the original size

    The size on the door placard, not necessarily what is fitted now.

  2. Step 2: Enter the fitted size

    What is actually on the car.

  3. Step 3: Pick a speed to check

    Use a speed you actually travel at — the absolute error grows with speed.

  4. Step 4: Consider recalibration

    Many vehicles can be recalibrated for rolling diameter through the body module or a tuner.

Example usageWorked figures06

Example usage

A modest increase
Original 225/60R16 at 26.63 in, fitted 235/60R16 at 27.10 in. At an indicated 70 mph you are actually doing 71.2 — a 1.2 mph under-read.
A lifted truck
265/70R17 at 31.61 in replaced by 285/70R17 at 32.71 in. Indicated 70 is really 72.4 mph, and the odometer under-records by 3.5%.
Smaller winter tyres
Fitting a 3% smaller set means an indicated 70 is really 67.9 mph — the speedometer now over-reads, which is the safer direction to be wrong.
Frequently asked questionsCommon questions07

Frequently asked questions

Do bigger tires make the speedometer read low?

Yes. A larger tyre covers more ground per revolution, so the vehicle travels further than the sensor believes and you are going faster than the dial shows.

Does the odometer change too?

By exactly the same proportion. Taller tyres mean recorded mileage accumulates more slowly than actual distance, which affects service intervals and resale figures.

Can I recalibrate my speedometer?

Many modern vehicles can be recalibrated for a different rolling diameter through the body control module or an aftermarket tuner. Whether yours can is model-specific.

Is my speedometer accurate from the factory?

Usually it reads slightly high. Regulations in most markets allow a speedometer to over-read but never to under-read, so manufacturers build in a safety margin.

How much error is acceptable?

Keeping tyre diameter within about three per cent keeps the error small enough to ignore in practice. Beyond that it becomes worth recalibrating.

Related toolsElsewhere on the site08

Browse every tool in Automotive Calculators.