Upsizing your tyres changes how far the car travels per wheel revolution, and your speedo doesn’t know you’ve done it. The result is a reading that’s optimistic in the wrong direction. Whether you legally and practically need to fix it depends on how big the jump is. Here’s the plain-English version for Australian drivers in 2026.

What does the law actually require?

Australia’s speedometer standard is ADR 18. Two rules matter: the speedo must never indicate less than your true speed, and it may over-read by no more than 10% plus 4 km/h. Written out, indicated speed ≤ (true speed × 1.10) + 4 km/h. So at a true 100 km/h the speedo can legally show anywhere from 100 up to 114 km/h — but never 99. Both CarsGuide and Auto Expert confirm this tolerance.

Why do bigger tyres make the speedo under-read?

A taller tyre covers more ground per revolution. For a given actual speed the wheels turn more slowly, so the speedo — which counts revolutions — thinks you’re going slower than you are. That’s the dangerous direction: your speedo might show 100 when you’re really doing 105, quietly pushing you over the limit. This is the opposite of worn tyres, which shrink and make the speedo over-read. The underlying maths is the tyre’s rolling diameter, and we cover the mechanism in how tyre size affects speedometer accuracy.

How much upsize is too much?

As a guide, here’s how common steps shift the reading:

Because factory speedos are usually set a few per cent fast to start with, a small upsize often just eats into that built-in margin and keeps you legal. A big jump can flip the speedo into under-reading territory — the illegal side. The detail on acceptable margins is in speedo error after upsizing: how much is too much.

How is a speedo recalibrated?

There are three common routes, depending on your vehicle:

• Tuner / handheld programmer: On many modern 4WDs the speedo factor is a software setting a tuner can adjust for the new tyre size — often the cheapest fix.
• Recalibration module: A plug-in device corrects the signal to the cluster where the ECU can’t be reflashed.
• Diff re-gear: If you’re re-gearing anyway to suit big tyres, the ratio change can restore both drivability and speedo accuracy.

Get it done by an auto electrician or driveline specialist, then confirm with a GPS or a known-distance road test. If you’re upsizing as part of a lift, line it up with the right tyres for a two-inch lift and the other jobs that come with it.

What else changes when you upsize?

Recalibration fixes the reading, not the side effects. Bigger tyres cost more fuel — see the 33-inch fuel-economy guide — and can affect driveline components. They don’t automatically void your warranty, but an uncorrected speedo and related data don’t help; see do bigger tyres void your warranty. If you’ve done a GVM upgrade or run heavily aggressive rubber, confirm the whole package is compliant with a licensed certifier.

Frequently asked questions

Is driving with an under-reading speedo illegal? A speedo that reads below true speed breaches ADR 18, and you’re responsible for your actual speed regardless of what the dial shows.

Does ABS or the ECU get confused by bigger tyres? They can — wheel-speed data feeds several systems, which is why correcting the tyre factor matters beyond just the dial.

Can I just do mental maths instead? You can estimate, but recalibration is safer and keeps you compliant; it’s cheap on many vehicles.

Do I need it for a small all-terrain upsize? Often no — a 1–3% change usually stays within tolerance. Verify with a GPS check.

Bottom line: Bigger tyres make your speedo under-read, and ADR 18 says it must never read below your true speed. A small upsize usually stays legal thanks to the factory margin; a big jump (around 35s) typically needs recalibration via a tuner, module or re-gear. Fix it, then verify against GPS.

ADR figures are current as a guide for 2026. Confirm recalibration and compliance with an auto electrician, driveline specialist or your state road authority.