How is speedometer error calculated?

The formula is: error (%) = ((indicated speed - actual speed) / actual speed) x 100. A positive result means the speedo reads higher than your true speed (over-reading). A negative result means it reads lower (under-reading), which is the more dangerous case.

Why do speedometers over-read by design?

EU Directive 97/39/EC and UK law require that a speedometer must never under-read. Manufacturers therefore calibrate instruments to read 0 to +10% above true speed. A reading of +5% on a 70 mph motorway means you are actually doing about 66.7 mph — safely legal but not wasteful on fuel.

How do I measure my actual speed accurately?

The best free method is a dedicated GPS speedometer app (e.g. Speedview, GPS Speedometer) on a modern smartphone. GPS-derived speed is accurate to ±1 mph or better in open sky. Police radar guns and Vascar equipment are also reliable. Avoid relying on satellite navigation map-speed readouts, which lag and average over distance.

Does changing tyre size affect my speedometer?

Yes. The ECU and speedo count wheel rotations and multiply by the factory tyre circumference. If you fit larger tyres (bigger overall diameter), each revolution covers more ground than the ECU expects, so the car travels faster than the speedo shows — the speedo under-reads. Smaller tyres have the opposite effect. The tire-size section of this calculator uses the P-metric formula: overall diameter (in) = rim diameter + 2 x (section width mm x aspect ratio / 100) / 25.4.

What is a safe speedometer error to ignore?

Under EU rules a reading of 0 to +10% is legal. In practice, most factory speedos read +3 to +7%. An error outside this band — especially under-reading — warrants investigation. Common causes: worn tyres (smaller diameter), after-market alloys with different offsets, or a faulty vehicle speed sensor (VSS).

Can a speedometer error cause a speeding fine?

Legally, in the UK courts have consistently held that an over-reading speedometer is not a defence to a speeding charge — the offence is travelling above the limit, regardless of what the dash shows. However, a calibrated GPS log can be used in mitigation to demonstrate the error. Underspeeding due to a faulty under-reading speedo is the real safety risk: you may be 6-7 mph faster than the number on the dash.

Speedometer Error Calculator

Your car’s speedometer is almost certainly not perfectly accurate — and that is entirely by design. UK and EU regulations allow it to read up to 10% higher than your true speed, but it must never read lower. In practice, most factory instruments read between 3% and 7% high, meaning that when your dash reads 70 mph on a motorway, you are typically doing 65–68 mph in reality. This calculator lets you measure exactly how far off your speedo is, predict your true speed at any limit, and understand how a tyre-size change shifts the error.

How speedometer error works

A modern vehicle speed sensor (VSS) counts the number of teeth passing a hall-effect sensor on the gearbox output shaft or ABS ring. The ECU multiplies that pulse rate by a calibration constant derived from the factory tyre’s rolling circumference. If the tyre diameter is larger than expected, each revolution carries the car further than the ECU calculates — the speedo under-reads. If the diameter is smaller (worn tread, narrower profile), the speedo over-reads even more than usual.

The error formula is straightforward:

Error (%) = ((Indicated - Actual) / Actual) x 100

A result of +5% means your speedo reads 5% high. At an indicated 60 mph, your true speed is 60 / 1.05 = 57.1 mph.

To go the other way — find your true speed from a known error:

Actual = Indicated / (1 + Error/100)

Or to find what your speedo would show if your true speed and the error are both known:

Indicated = Actual x (1 + Error/100)

The tyre-size formula

When you change tyre size, the overall diameter changes. For a P-metric tyre (e.g. 215/55 R17):

Overall diameter (in) = Rim (in) + 2 x (Section width mm x Aspect ratio / 100) / 25.4

The circumference is then pi x diameter. The speedometer error introduced by the tyre change is:

Tire error (%) = ((Indicated - True speed) / True speed) x 100

where True speed = Indicated x (New circumference / Original circumference).

Worked example

Your dashboard reads 70 mph. A GPS app shows 66.5 mph.

Error = ((70 - 66.5) / 66.5) x 100 = +5.26% (over-reading — within legal tolerance)
At an indicated 30 mph limit: true speed = 30 / 1.0526 = 28.5 mph (you are fine)
At an indicated 20 mph limit: true speed = 20 / 1.0526 = 19.0 mph (also fine)

Now suppose you swap from 205/55 R16 to 225/55 R16:

Original diameter: 16 + 2 x (205 x 0.55 / 25.4) = 24.88 in
New diameter: 16 + 2 x (225 x 0.55 / 25.4) = 25.74 in
Diameter growth: +0.86 in (+3.5%)
At indicated 70 mph, true speed: 70 x (25.74 / 24.88) = 72.4 mph — the speedo now under-reads by 3.3%

That 3.3% under-read, on top of the original +5.26% over-read, brings the combined error to roughly +1.8% — still legal, and in fact closer to reality.

Legal context

In the UK, the Road Vehicles (Construction and Use) Regulations 1986 require that the speedometer must not under-read. The EU Directive 97/39/EC (retained in UK law post-Brexit) sets the tolerance band at 0 to +10% of true speed. Manufacturers uniformly target the positive end of this range as a safety margin. Police speed equipment (radar, laser, Vascar) is calibrated to measured road distance and is unaffected by your speedo error — so a faulty instrument provides no legal defence.