Why a taller tyre
makes the needle lie low
A speedometer does not measure speed. It counts wheel revolutions and multiplies by a circumference it was told about years ago in a factory. Change the tyre and you change the multiplier — but nobody tells the instrument.
10% + 4 km/h Under UN ECE Regulation 39, a speedometer may read high by up to 10% of true speed plus 4 km/h. It may never read low. Not by 1%, not by anything.
That asymmetry is the whole story. Going up a tyre size makes the needle read low — the one direction the rule gives you no room in at all — while going down makes it read high, into the tolerance that already exists.
The arithmetic, once
The instrument was calibrated for a rolling circumference. Fit a tyre with a different circumference and every revolution now covers a different distance, in exact proportion:
true speed = indicated speed × (new overall diameter ÷ old overall diameter)
That is all of it. A 235/45R17 on a car built for 225/45R17 is +1.42% on diameter, so an indicated 100 km/h is really 101.4 km/h. The odometer drifts by the same proportion, and so does anything derived from it — service intervals, fuel economy, a lease's mileage allowance.
The rule where you are
Most of the world runs the same regulation. The United States is the exception, and not in a comforting direction.
| Market | Instrument | What it says |
|---|---|---|
| New Zealand | UN ECE R39, via the Land Transport Rule: Vehicle Standards Compliance | The speedometer must not read below true speed, and may read up to 10% + 4 km/h high. |
| Australia | ADR 18/03, which adopts UN ECE R39 | Same limits: never low, at most 10% + 4 km/h high. |
| United Kingdom | UN ECE R39 (retained EU type approval) | Same limits. This is why UK cars famously read a few mph high from the factory. |
| European Union | UN ECE R39 | Same limits across all member states. |
| United States | FMVSS 101 requires a speedometer but sets no accuracy tolerance | There is no federal accuracy standard for passenger-car speedometers. Enforcement is a state matter and in practice the tyre is your problem, not the instrument's. |
SOURCE·UN ECE Regulation 39 (speedometer and odometer equipment)·as adopted into NZ, AU, UK and EU vehicle standards; US position is FMVSS 101·REVIEWED ETRTO STANDARD
What the ceiling actually looks like
The highest reading the dial is permitted to show at each true speed.
| True speed | Lowest legal reading | Highest legal reading | The allowance |
|---|---|---|---|
| 50 km/h | 50 km/h | 59 km/h | +9 km/h, and nothing below |
| 60 km/h | 60 km/h | 70 km/h | +10 km/h, and nothing below |
| 80 km/h | 80 km/h | 92 km/h | +12 km/h, and nothing below |
| 100 km/h | 100 km/h | 114 km/h | +14 km/h, and nothing below |
| 110 km/h | 110 km/h | 125 km/h | +15 km/h, and nothing below |
A factory speedometer typically sits a few km/h into that allowance already. Fitting a tyre 3% taller consumes roughly 3 km/h of it at 100 km/h — which is why a modest-looking change can push a car from "reads high, as designed" to "reads low, which is not permitted at all".
The three things that also drift
- The odometer. Same percentage, permanently. On a 3% taller tyre, 100,000 km displayed is 103,000 km travelled — and it never comes back.
- ABS and stability control. Both compare wheel speeds against a reference derived from rolling radius. Manufacturers validate those systems against a diameter, which is where the ±3% convention comes from in the first place.
- Fuel economy read-outs. Litres per 100 km is distance-derived, so an optimistic odometer makes the car look thirstier than it is, and a pessimistic one flatters it.
Speedometer error — the questions that follow
Is a speedometer allowed to read low?
No — not in any market that adopted UN ECE Regulation 39, which includes New Zealand, Australia, the United Kingdom and the whole European Union. The indicated speed must never be below true speed. It may be above it, by up to 10% + 4 km/h. The rule is deliberately one-sided.
Why do new cars read high from the factory?
Because the manufacturer has to guarantee the needle never reads low across tyre wear, tyre pressure, load and every approved fitment. The cheapest way to guarantee that is to bias the instrument high and spend part of the 10% + 4 km/h allowance up front. A worn tyre is slightly smaller, which pushes the reading even higher — so the bias also has to survive a set of tyres reaching the wear bars.
How much does a taller tyre change my speedometer?
By exactly the percentage change in rolling diameter. Fit a tyre 2% taller and the needle reads 2% low at every speed: 100 km/h indicated becomes 102 km/h true. There is no correction and no calibration on most cars — the instrument simply keeps counting revolutions against the circumference it was built for.
Does the United States have a speedometer accuracy law?
Not a federal one. FMVSS 101 requires that a speedometer be fitted and legible but sets no accuracy tolerance for passenger cars. That does not make an inaccurate speedometer a good idea — it means the consequence lands on you rather than on the instrument.
Work out your own change on the size calculator, or start from a size on the size index. The geometry here is exact; whether a tyre fits YOUR car is not something geometry can answer. Arch clearance, suspension travel, offset, load rating and your vehicle's own placard all decide that, and none of them are in a size string. The ±3% rule quoted throughout is an industry convention, not a law. Confirm any change against the tyre placard on your door pillar and with a tyre fitter before you buy.