Speedometer Accuracy Law UK: What the Law Requires in the UK & USA

Speedometer accuracy law in the UK sets the limits on how far a speedometer may be out, and it is the legal background to every calculation on this site. In the UK the rule comes from the Road Vehicles (Construction and Use) Regulations and the equivalent European type-approval standard: a speedometer must never show less than the true speed, and it may show more than the true speed by up to 10% plus 4 km/h. In the USA there is no federal standard that sets a speedometer accuracy tolerance in the same way, so the position is different and this page explains both. The figures here describe the law as it stands and limits change, so check the current rules for the jurisdiction before relying on them.

For speedometer accuracy law uk, the point is this: The practical consequence is that manufacturers deliberately calibrate speedometers to read slightly high, so a dial that reads 70 mph is usually showing a true speed a little below 70 mph. That built-in over-read is why a small tyre change often leaves the dial still reading high rather than suddenly reading low — and why the dangerous direction is fitting larger tyres, which eats into the safety margin and can push the dial into under-reading. The worked example below uses 205/55R16 replaced by 215/55R16, a +1.74% change, and the tables show the indicated-versus-actual speeds at 30, 50 and 70 mph.

This is a calculation, not a safety approval. Always check your vehicle's door-jamb placard, load index and speed rating, and confirm fitment with a tyre professional before fitting a different tyre size.

How the speedometer error is calculated for speedometer accuracy law uk

For speedometer accuracy law uk, the point is this: The arithmetic is short. First find the percentage change: error % = (new diameter − old diameter) ÷ old diameter × 100. Then find the actual speed: actual speed = indicated speed × (new diameter ÷ old diameter). Both formulas use the nominal new-tyre diameters from the size codes, and the tables in this article use those figures throughout. A positive result means the replacement tyre is larger, so the true speed is higher than the indicated speed and the odometer under-records; a negative result means the replacement is smaller, so the true speed is lower than the indicated speed and the odometer over-records. The same percentage applies at every speed, so one calculation covers the whole speed range.

speedometer accuracy law uk: What the speedometer actually measures

Taking speedometer accuracy law uk as the case in point: The speedometer does not measure speed directly. It counts wheel revolutions — from a sensor on the gearbox output, a wheel-speed sensor or the ABS system, depending on the vehicle — and multiplies by the rolling circumference the manufacturer programmed in. That circumference belongs to the original tyre size, so fitting a different diameter changes the distance covered per revolution while the speedometer keeps using the old figure. The result is a fixed percentage error: if the new tyre is 2% larger in diameter, the actual speed is 2% higher than the indicated speed at every speed, and the odometer under-records by the same 2%. The error does not grow or shrink with speed; it is the same percentage at 30 mph and at 70 mph, which is what the tables below show.

Worked example: 205/55R16 to 215/55R16 against the law for speedometer accuracy law uk

Taking speedometer accuracy law uk as the case in point: 205/55R16 measures 631.9 mm and 215/55R16 measures 642.9 mm, a +1.74% change. If the speedometer read exactly true on the original tyres, it would read 98.26 mph at a true 100 mph after the change — it under-reads, which the UK rule forbids. In practice most speedometers read slightly high from the factory, so a +1.74% change often still leaves the dial reading high, but the margin is smaller than it was. The calculation shows why the direction matters: a change that makes the dial read low is the one that risks breaking the law, while a change that makes it read high stays inside the rule as long as the over-read stays within 10% plus 4 km/h.

speedometer accuracy law uk: Indicated vs actual speed: the worked example

Error: +1.74%. Actual speed = indicated speed × (642.9 ÷ 631.9). That is the point this page works through for speedometer accuracy law uk.

Indicated speed Actual speed Difference
30 mph 30.5 mph +0.5 mph
50 mph 50.9 mph +0.9 mph
70 mph 71.2 mph +1.2 mph

On this page, speedometer accuracy law uk is the case in point: In the UK the construction rule is that a speedometer must never show less than the true speed and may show up to 10% more than the true speed plus 4 km/h, and that limit applies to the vehicle as type-approved. In the USA there is no federal accuracy standard for speedometers that sets a tolerance in the same way, so the position is left to manufacturers and to state rules, and a dial that under-reads is a problem for the driver regardless of jurisdiction because it means travelling faster than believed. Limits change, so check the current rules for the jurisdiction before relying on them, and remember that the odometer error follows the same percentage.

Common causes of a wrong speedometer for speedometer accuracy law uk

For speedometer accuracy law uk, the point is this: Several things can put a speedometer out, and they are worth separating because the fixes differ. The first is tyre diameter: any change from the size the vehicle was calibrated for produces a fixed percentage error, and it is the cause this article is about. The second is wear and pressure: a worn tyre is smaller than a new one and an under-inflated tyre has a smaller effective rolling radius, so both make the dial read slightly high. The third is a previous recalibration — a programmer setting or a speedometer gear fitted for an earlier tyre size that no longer matches what is on the car. The fourth is a fault: a failing wheel-speed sensor, a damaged reluctor ring or, on older cars, a worn cable, which usually shows as a needle that jumps, sticks or reads inconsistently rather than a steady percentage error. A steady percentage error points at calibration; an erratic needle points at a fault.

How to fix a speedometer error for speedometer accuracy law uk

Taking speedometer accuracy law uk as the case in point: Fixing a speedometer error means telling the vehicle the new tyre diameter, and the method depends on the vehicle. On many modern cars a handheld programmer or a diagnostic tool can rewrite the tyre-size setting in the body or engine control module, and the speedometer and odometer then read correctly for the new size. On older vehicles with a mechanical speedometer, the fix is a different speedometer drive gear in the gearbox, chosen so the gear ratio compensates for the diameter change. Some vehicles need a dealer visit because the setting is locked behind manufacturer software, and some need an inline speedometer calibrator fitted in the sensor wiring. Whichever method is used, the correct one depends on the vehicle, so the owner's manual or a specialist should be consulted before buying a tool — and the error should be measured against GPS before and after, at a steady speed, to confirm the fix worked.

Common mistakes with speedometer error for speedometer accuracy law uk

On this page, speedometer accuracy law uk is the case in point: The most common mistake is assuming the speedometer was accurate before the tyre change. Most speedometers are deliberately calibrated to read slightly high — in the UK the law requires that a speedometer never under-reads, and it may over-read by up to 10% plus 4 km/h — so a car that read 2% high on the original tyres and now reads 2% low on larger tyres has actually changed by 4% from what the driver was used to. The second mistake is checking the speedometer against another car's speedometer instead of against GPS: two dials can share the same error. The third is testing at one speed only and assuming the error is a fixed number of mph — it is a percentage, so the mph error grows with speed. And the fourth is treating a calculation as a reason to ignore the limit: if the dial reads low, the car is travelling faster than the driver thinks, and that is the direction that risks a speeding offence.

speedometer accuracy law uk: Related pages

Frequently asked questions

How accurate does a speedometer have to be in the UK?

In the UK a speedometer must never show less than the true speed, and it may show up to 10% more than the true speed plus 4 km/h. That is the construction and type-approval rule, and it is why manufacturers calibrate dials to read slightly high. Limits change, so check the current rules before relying on them. That is the point this page works through for speedometer accuracy law uk.

Is there a speedometer accuracy law in the USA?

There is no federal standard in the USA that sets a speedometer accuracy tolerance in the same way as the UK rule, so the position is left to manufacturers and to state rules. A dial that under-reads is a problem for the driver in any jurisdiction, because it means travelling faster than believed. That is the point this page works through for speedometer accuracy law uk.

Can a tyre change make my speedometer illegal?

Taking speedometer accuracy law uk as the case in point: It can, in the UK, if the change makes the dial under-read — the law requires that it never shows less than the true speed. A larger tyre moves the dial in that direction, so a big diameter increase can push a dial that was reading slightly high into under-reading. Work out the error percentage from the two sizes before fitting.

How do I check my speedometer against GPS?

Taking speedometer accuracy law uk as the case in point: Drive at a steady speed on a straight, level road and compare the dial with a GPS speed reading. GPS is least reliable at low speed, while accelerating or braking, and on hills or in tunnels, so take the reading at a steady cruise. A difference that is the same percentage at several speeds is a calibration error; a needle that jumps or sticks is a fault.

Does tyre wear affect the speedometer?

Taking speedometer accuracy law uk as the case in point: Yes, slightly. A worn tyre has a smaller diameter than a new one, so the speedometer reads slightly high as the tread wears — typically well under 1% over the life of the tyre. Under-inflation has the same direction of effect by reducing the effective rolling radius. Both are small compared with a size change, but they are real.