Tyre size calculator
Compare the size on your car with the one you are thinking of fitting. Pick both sizes to see how much taller or shorter the new tyre is, and what your speedometer would read after the change.
How the numbers are worked out
The sidewall height is the width multiplied by the profile: 45% of 225 mm is about 101 mm. The overall diameter is the rim, converted at 25.4 mm to the inch, plus a sidewall above and below it. The circumference is that diameter multiplied by π, and a kilometre divided by the circumference gives the revolutions per km.
These are nominal figures from the size code. A real tyre measures slightly differently depending on the brand, the tread left on it and its pressure.
Why the speedometer changes
The speedometer works from how fast the wheels turn, and it was set up for the size that came on the car. A taller tyre travels further with every turn, so the car goes faster than the speedometer shows. A shorter tyre does the opposite.
How to read your tyre sizeBefore you change size in NSW
Load rating
Every replacement tyre must have a load rating equal to or higher than the tyres the manufacturer fitted. You will find it on the tyre placard.
Inside the guards
The wheel and tyre must stay within the bodywork or mudguards, including any flares, when the wheels point straight ahead.
Bigger changes need a certificate
On vehicles up to 4.5 tonnes, tyres that change the overall diameter by more than 7% of the largest diameter on the tyre placard, or that affect speedometer compliance, need a compliance certificate from a licensed certifier.
Sources: Transport for NSW VSI 09, alternative wheels and tyres (Rev 5) · Transport for NSW VSI 06, light vehicle modifications (Rev 4, July 2026)
Ready to change size?
Read us both sizes on the phone and we can quote you for either. Whatever the size, we match or exceed the original load index on every fitting.
Call 1300 689 868