How to Verify Tyre Fitment Before You Buy in Australia
To verify tyre fitment, match four things against your vehicle’s tyre placard: the size, the load index, the speed rating and the overall diameter. Then check the date code on the tyre itself. If all of them line up, the tyre fits and is legal to run. If any one is lower or off, you stop and ask before the tyre is on the rim.
This guide walks through each check in the order a fitter would do it, with the Australian rules behind them and the mistakes we see most often.
TL;DR
- Your tyre placard is the starting point for size, load index, speed rating and cold pressure. Not the number stamped on the sidewall.
- A replacement tyre must meet or exceed the manufacturer’s load and speed ratings. Lower is never acceptable.
- For standard cars, overall diameter can’t grow more than 15 mm or shrink more than 26 mm from the original. The popular “3% rule” is a guideline, not the law.
- Check the four-digit date code before you pay. A tyre can be new to you and still be several years old.
- When a size is unusual (bigger wheels, 4WD, towing), have the fitter confirm it before the booking is locked in.
Table of Contents
- Start with the placard, not the old tyre
- Decode the size on the sidewall
- Load index: the check people skip
- Speed rating: never go lower
- Overall diameter: what the law actually limits
- Check the age of the tyre
- Special cases that change the answer
- A five-minute checklist before you buy
- Our view on fitment
- Book a verified fit with Treadivo
- Frequently asked questions
- Sources
Start with the placard, not the old tyre
The tyre currently on your car is only a guide. The previous owner may have fitted something different, and a mismatched tyre on the wheel tells you nothing about what the manufacturer intended.
The Tasmanian Department of State Growth guidance describes the tyre placard as the reference for tyre size, load index, speed rating and recommended cold inflation pressure. It’s usually on the driver’s door jamb, and on some cars inside the glovebox or the fuel filler door.
Write down, or photograph, these items from the placard:
- Front and rear tyre sizes (they can differ, especially on performance cars and some SUVs).
- Load index and speed rating, shown together as something like 91V.
- Recommended cold pressure for front and rear, and for loaded versus unladen if listed.
A word on pressure. The maximum figure moulded into the sidewall is a limit for the tyre structure. It isn’t your car’s pressure. Use the placard figure, set when the tyres are cold.
No placard, or it’s unreadable? Check the owner’s manual, or have a fitter look the vehicle up. Our guide to finding the right tyres by rego, ADR and same-day fitting covers the lookup route.
Decode the size on the sidewall
A typical size reads 235/50R19 99V. Each part has a job.
| Part | Example | What it tells you |
|---|---|---|
| Section width | 235 | Width of the tread area in millimetres |
| Aspect ratio | 50 | Sidewall height as a percentage of width |
| Construction | R | Radial |
| Rim diameter | 19 | Wheel size in inches |
| Load index | 99 | Maximum load per tyre (a code, not kilograms) |
| Speed rating | V | Maximum speed the tyre is rated for |
Two tyres can share a rim diameter and still differ in overall height. A 265/60R18 and a 265/65R18 both fit an 18-inch wheel, but the second is noticeably taller. That’s why rim size alone never confirms fitment.
If you want to run the numbers yourself, our tyre size calculator shows overall diameter, sidewall height and the difference against your original size. You can also browse the tyre sizes directory to see which sizes exist for a given rim.
Load index: the check people skip
Load index is a code. The Northern Territory Government publishes a table converting each index to a kilogram load per tyre, and notes that load and speed rating appear on the tyre sidewall. For example, an index of 91 corresponds to 615 kg per tyre, as we explain in our piece on what Australian drivers must check with load index.
The rule is simple: the replacement must meet or exceed the load index on the placard. The Tasmanian guidance above says these ratings are checked during roadworthy inspections, and a vehicle that doesn’t meet them can be unroadworthy and ineligible for registration.
Where this goes wrong in practice:
- Cheap passenger tyres on an SUV. The size matches, the load index is lower. It looks fine and isn’t.
- Towing and loaded utes. If you carry weight regularly, a tyre at exactly the placard minimum leaves no margin. Talk to the fitter about a higher-rated option.
- Commercial and light truck sizes. Many sizes come in more than one load range. Check the marking, not just the numbers in the size.
Speed rating: never go lower
The speed rating letter sits after the load index. Same principle: match or exceed the placard. Fitting a lower rating than the manufacturer specified is a roadworthy failure, not a matter of taste. Our note on never fitting a lower tyre speed rating goes through the letters and what each means.
Higher is fine from a compliance view, but it can change ride and price. A V-rated tyre on a car that only needs T will usually cost more and may feel firmer.
Overall diameter: what the law actually limits
This is where most online advice goes loose. You’ll hear that you can change tyre size by 3%. That’s a rule of thumb for keeping the speedometer reasonably accurate. It isn’t what determines legality.
For modified vehicles, the National Code of Practice for Light Vehicle Construction and Modification, Section LS (Suspension and Steering), sets diameter limits. As published in that document, a standard passenger car’s overall tyre diameter can’t increase by more than 15 mm or decrease by more than 26 mm from the manufacturer’s size. For off-road 4WD passenger vehicles, the allowable increase is up to 50 mm. Rules can differ with other modifications, such as lifts, so confirm for your state and your setup.
A worked example
Take a 235/50R19. Sidewall height is 50% of 235 mm, which is 117.5 mm. Two sidewalls plus a 19-inch rim (482.6 mm) give an overall diameter of about 717.6 mm.
Now consider 245/45R19. Sidewall height is 110.25 mm, so overall diameter is about 703.1 mm. That’s roughly 14.5 mm smaller, well inside the 26 mm decrease limit for a car.
Swap to 245/50R19 instead and the sidewall is 122.5 mm, giving about 727.6 mm. That’s roughly 10 mm taller, inside the 15 mm limit. Go to 255/50R19 and you reach about 737.6 mm (sidewall 127.5 mm), which is around 20 mm taller and over the limit for a standard car.
The numbers look small, but they add up quickly. Our NCOP limits guide to calculating tyre size shows the method with more sizes.
Width is a separate question
Diameter being legal doesn’t mean the tyre clears. A wider tyre can rub on the guard, suspension arms or inner liner, especially on full steering lock. The wheel’s width and offset matter too. If you’re changing wheels, not just tyres, get the fitter to check clearance before you order.
Check the age of the tyre
Fitment isn’t only about size. Look for the DOT code on the sidewall. The last four digits give the week and year of manufacture: 2824 means week 28 of 2024.
The Tasmanian guidance notes that rubber degrades with heat and UV exposure, that tyres of five years or more should be professionally inspected even with good tread, and that industry experts advise removal from service at ten years. Some vehicle manufacturers set their own limits, and those take precedence. We lay out the difference between law and advice in our guide to tyre age limits in Australia.
Practical step: when you book, ask for the manufacture date on the tyres you’ll receive. A reputable supplier shouldn’t mind.
Special cases that change the answer
4WDs and off-road
Off-road vehicles get a larger allowable diameter increase, but that’s within a regulatory framework, not a free pass. The 4WD publication Unsealed 4X4 discusses how off-road tyres are typically taller and why the regulations still matter when you go bigger. Check speedometer error, gearing effect and clearance at full suspension travel.
Electric vehicles
EVs often specify tyres with a particular load rating and construction because of vehicle weight. Match the placard exactly. Our guide to EV tyres in Australia covers the common mistakes.
Staggered fitments
If the front and rear sizes differ, check each axle separately. Rotating tyres front to back isn’t possible on these cars, so don’t assume one set of four.
A five-minute checklist before you buy
- Photograph the placard.
- Compare the candidate tyre’s size against both axles.
- Confirm load index is equal or higher.
- Confirm speed rating is equal or higher.
- Calculate overall diameter and compare it with the original.
- If you’ve modified the car (lift, bigger wheels, towing), ask the fitter to confirm in writing.
- Ask for the date code.
If a size doesn’t appear when you search by car model, don’t push on with a near match. Go back to the placard and try by size.
Our view on fitment
Most fitment problems aren’t dramatic. They’re small gaps between what someone assumed and what the placard says: a tyre one load index lower, a rear size mistaken for the front, a date code nobody looked at. The fix is cheap if you catch it before the tyre is on the rim and annoying if you catch it after.
That’s why we built Treadivo around searching by car model or tyre size, showing itemised pricing up front and having a local fitter confirm the booking. The AR preview helps you judge how a tyre looks, but it isn’t a substitute for the placard. Always check the numbers first, then the looks. If you’re between two sizes, the one that matches the placard is the right start.
— Areesh
Book a verified fit with Treadivo
When you’ve confirmed your size, you can shop tyres online by car model or tyre size, pick a local fitter and complete checkout in about three minutes. Fitters are available around Perth and Melbourne, with same-day fitting confirmation. Pricing is itemised, with no hidden fees.
Frequently asked questions
How can I check my wheel fitment?
Start with the placard for tyre size, load and speed rating. Then compare the new tyre’s overall diameter against the original and confirm the wheel’s width and offset leave clearance on full lock. If you’ve changed wheels or suspension, ask a fitter to check before you buy.
What is the 3% rule for tyres?
It’s a rule of thumb for keeping speedometer readings and handling close to original. It isn’t the legal limit. For standard cars the code sets a maximum diameter increase of 15 mm and decrease of 26 mm, and up to 50 mm increase for off-road 4WDs.
Are 7-year-old tyres too old?
Not automatically, but they should be inspected. Guidance says tyres five years or older deserve a professional check even with good tread, and ten years is the point experts advise removal. Your vehicle manufacturer may set a shorter limit.
Can I fit a tyre with a lower load index or speed rating?
No. Replacement tyres need to meet or exceed the manufacturer’s ratings on the placard. Lower ratings can make the vehicle unroadworthy and affect registration.
Where do I find my tyre size?
On the placard (usually the driver’s door jamb), on the sidewall of the current tyres, or in the owner’s manual. If they disagree, trust the placard.
Sources
- Tasmanian Department of State Growth: tyre load and speed rating inspection guidance
- National Code of Practice, Section LS Suspension and Steering (Queensland TMR)
- NT.GOV.AU: Tyre load and speed rating
- Unsealed 4X4: Bigger tyres, bigger problems?