Tyre size calculator
Everything your tyre is telling you.
Type the size from your sidewall or door-jamb placard. Hover or tab through each part to see what it means.
Plus sizing
Tyre size comparison and speedo error
Changing wheel or tyre size moves your overall diameter, which your speedometer, odometer, ABS and stability control all rely on.
Diameter change
+1.6%
Speedo reads 100 km/h, you’re doing
101.6
Within the usual ±3% rule of thumb, so ABS, stability control and the speedometer should cope. Check clearance to the guards and suspension at full lock and full compression, and that the new width suits your rim. With a bigger tyre your speedo under-reads: you're travelling faster than it shows.
| Current | New | Change | |
|---|---|---|---|
| Diameter | 637 | 647 | +10 |
| Sidewall | 90 | 82 | -8 |
| Width | 225 | 235 | +10 |
| Revs/km | 500 | 492 | -8 |
Current, with the new size dashed
Tyre age
Tyre date code: how old is that tyre?
Rubber ages whether or not the tyre is used. A tyre with plenty of tread can still be too old to trust, and that includes the spare.
Look for “DOT” on the sidewall followed by letters and numbers, often on one side only. The final four digits, usually in an oval, are the week and year of manufacture: 2423 means week 24 of 2023.
3.3 years old
Made in week 24 of 2023
Under five years old. Keep checking tread depth and look for cracks in the sidewall.
Tread depth
Minimum tread depth and when to replace
1.5 mm
Legal minimum in Australia
The tread-wear indicator bars moulded into the grooves sit at this depth. At 1.6 mm in the UK and EU, 2/32 in (1.6 mm) in most US states.
3 mm
Replace for the wet
Below about 3 mm, stopping distances in the wet grow quickly as grooves can't clear water. Most tyre makers suggest replacing here.
4 mm
Winter tyres
Winter and all-weather tyres lose much of their snow grip below about 4 mm.
Questions
Tyre sidewall questions, answered
How do I read a tyre size?
Take 245/40 ZR18 97Y. 245 is the width in millimetres. 40 is the sidewall height as a percentage of that width. Z means rated above 240 km/h and R means radial construction. 18 is the rim diameter in inches. 97 is the load index (730 kg per tyre) and Y is the speed rating (up to 300 km/h).
How do I know how old my tyres are?
Find the DOT code on the sidewall; it may be on one side only. Its last four digits are the week and year of manufacture, so 2423 means week 24 of 2023. Tyres made before 2000 used three digits.
How often should tyres be replaced?
By tread, at the legal minimum at the latest (1.5 mm in Australia) and ideally around 3 mm, where wet braking starts to fall away. By age, most tyre makers recommend a yearly inspection from five years old and replacement at ten years regardless of tread, including the spare.
What is the minimum legal tread depth in Australia?
1.5 mm across the tread, which is where the tread-wear indicator bars moulded into the grooves sit. The UK and EU use 1.6 mm, and most US states 2/32 inch.
What is a tyre load index?
A number standing for the maximum load one tyre can carry at its rated pressure. Load index 91 is 615 kg, 95 is 690 kg and 100 is 800 kg. Never fit a lower load index than your vehicle's tyre placard shows.
Can I fit tyres with a lower speed rating?
Generally no. Fit at least the load index and speed rating on your vehicle's tyre placard: a lower-rated tyre can make the car unroadworthy and may affect your insurance. Some jurisdictions allow exceptions, such as for winter tyres, so check with your transport authority first.
Will bigger tyres affect my speedometer?
Yes, if the overall diameter changes. A tyre 3% taller means you travel about 3% faster than the speedometer shows. Use the size comparison on this page to check, and keep the change within about 3%.
Reference
Tyre load index and speed rating charts
Tyre load index chart: maximum load per tyre
Tyre speed rating chart
Ratings assume a correctly inflated tyre at its rated load. Heat, underinflation and overloading all cut the real limit.