OEM Tyre Size: How to Find Your Car’s Original Size
Your car’s OEM tyre size is the size the manufacturer fitted and specified for your exact model and variant. In Australia the quickest place to read it is the tyre placard, usually on the driver’s door jamb. Check it against the code on your current tyres’ sidewall, because the previous owner may have changed sizes.
This guide shows where to find the original size, how to read it, and what to check if you want something different. It also covers why a tyre that fits on the rim can still be the wrong tyre.
TL;DR
- The original (stock) size is on the vehicle placard, in the owner’s manual, and usually on the tyres you bought the car with. Trust the placard over what is currently on the car.
- A size code gives width, aspect ratio and rim diameter. The load index and speed rating after it matter just as much.
- The popular “3% rule” is a screening heuristic, not a national legal test. Australian rules focus on load capacity, size suitability and roadworthiness.
- If your car is modified, ask a qualified fitter and your state or territory authority before you buy.
Table of Contents
- Where to find your OEM tyre size
- How to read a tyre size code
- Why the OEM size is the safe default
- What counts as a stock or equivalent size
- The “3% rule” in Australia
- Matching size is not enough: what else to check
- When to keep OEM and when to change
- Our perspective
- Buying the right size and getting it fitted
- FAQ
- Sources
Where to find your OEM tyre size
There are four reliable places. Start with the first and use the others to confirm.
- The vehicle placard. It is typically on the driver’s door jamb. Some cars have it in the glovebox or inside the fuel flap instead. It lists the manufacturer’s tyre size or sizes and the matching cold tyre pressures, front and rear.
- The owner’s manual. It usually repeats the placard and lists any alternative sizes for other variants, such as a base model on 16-inch wheels and a sportier grade on 18-inch.
- The sidewall of your current tyres. This tells you what is on the car now. It is the original size only if nobody has changed it.
- A fitment directory by make, model and year. Our tyre sizes directory lets you browse fitments. Cross-check any result against your placard, because variants within one model often differ.
One trap is worth knowing about. Many models were sold with two or three wheel and tyre options. Your friend’s identical-looking model may have a different original size. Always go by your own car’s placard, or by the compliance and build details for your specific variant.
How to read a tyre size code
Take a typical SUV size: 235/50R19 99V.
| Part | Example | What it means |
|---|---|---|
| Width | 235 | Section width in millimetres |
| Aspect ratio | 50 | Sidewall height as a percentage of the width |
| Construction | R | Radial |
| Rim diameter | 19 | Wheel diameter in inches |
| Load index | 99 | A number that maps to the maximum load per tyre |
| Speed rating | V | The maximum speed the tyre is designed for |
The first four items describe the physical size. The last two describe capability, and they are the ones people forget. A tyre with matching dimensions but a lower load index or lower speed rating is not like-for-like.
We cover the last two in detail elsewhere: our guide to tyre load index explains how the number translates into kilograms per tyre, and our piece on tyre speed ratings explains why you should never fit a lower rating than the original.
Light truck and 4x4 codes look different
Some 4x4 and light truck sizes use a flotation style, such as 315/75R16, and often carry a load range or ply rating. Here the load index (121 on a typical example) and the load range matter even more, since these vehicles carry gear, tow and travel on rough roads. Match or exceed the original load capability, not just the dimensions.
Why the OEM size is the safe default
The manufacturer chose the original size to suit the car’s weight, brakes, suspension geometry, wheel arches, speedometer calibration and stability systems. The placard combines that size with the right pressures.
Australian rules are built around this relationship. The intention of Australian Design Rule 24 (Tyre Selection) is to specify requirements for tyre selection appropriate to vehicle load capacity, rim size and speed characteristics. In other words, a tyre is judged on more than whether it physically mounts.
The Australian Light Vehicle Standards Rules 2015 also have a section on wheels and tyres, covering size and capacity. As we read it, wheels and tyres on an axle must be able to carry the load transmitted through that axle. That is why load capacity sits next to size on any sensible checklist.
Staying on the OEM size also keeps three practical things simple:
- Your speedometer and odometer stay accurate.
- Insurance and roadworthiness questions are easier, since nothing has changed from the manufacturer’s specification.
- Your tyre pressures on the placard apply directly.
What counts as a stock or equivalent size
People search for “stock”, “original” and “OEM” size as if they were different things. They are the same idea: the size the car left the factory with. Three cases come up often.
Same size, different brand or model. This is the easy case. Keep size, load index and speed rating at or above the original, and choose a tyre type that suits how you drive.
An alternative size listed by the manufacturer. If your manual lists a second size for your variant, that is also original. Check that your wheels match the size you choose.
A “plus size” or other change. Moving to a larger wheel with a lower-profile tyre is a modification. The goal is to keep the overall diameter close to the original so the speedometer, gearing and clearances stay close. Our tyre size calculator compares overall diameter, and our guide to calculating tyre size for Australia walks through the NCOP 11 limits.
The “3% rule” in Australia
You will see a rule of thumb that says you can change overall tyre diameter by up to 3% from the original. It is a useful screening test and it is not a law. Our reading of the research is that the national rules do not state a universal percentage. They require wheels and tyres of adequate size and load capacity, and tyre selection is governed by the applicable ADRs. State and territory rules can add conditions to modifications.
So treat 3% like this:
- If a new size is more than about 3% different, expect trouble: speedometer error, clearance problems or a problem at inspection.
- If it is within 3%, that does not approve it. It still has to carry the load, match the speed need, clear the guards and suspension through full lock and travel, and suit the wheel.
If you ask what the legal tyre size is, the honest answer is that there is no single legal size for Australia. The legal tyre for your car is one that meets the requirements for your vehicle, and for a stock car the placard size is the clean answer.
Worked example: 265/60R18 versus alternatives
Suppose your 4x4 wagon came with 265/60R18 and you wonder about a neighbouring size. Overall diameter is rim diameter plus two sidewalls. A 265/60R18 has a sidewall of 265 × 0.60 = 159 mm, so the overall diameter is about 18 × 25.4 + 2 × 159 = 775 mm. A change in width alone, such as to a 275, raises the sidewall too (275 × 0.60 = 165 mm), giving about 787 mm, around 1.5% larger. That sits within the screening range, but the wider tyre may rub, may need a different rim width, and the load index must still be right.
The arithmetic is easy. The clearance and legality are the hard part, which is where a fitter earns their keep. If you drive this size, our guide to 265/60R18 tyres has more.
Matching size is not enough: what else to check
A fitter will check more than the numbers on the sidewall. Before you buy a tyre in anything other than the placard size, run through this list.
- Load index. Equal to or higher than original. Heavier vehicles and those that tow need particular care.
- Speed rating. Equal to or higher than original.
- Rim width. Every tyre size has an approved range of rim widths. A tyre too wide for the rim, or too narrow, sits badly and handles differently.
- Clearance. Check for rubbing at full steering lock and over bumps, at the front and rear.
- Tyre type. A highway tyre and an all-terrain tyre suit different jobs. Our comparison of highway and all-terrain tyres covers the trade-offs.
- Age. Even the right size is a poor buy if it has sat for years. Read the date code, and see our guide on tyre age in Australia.
- Paperwork. If you modify wheel or tyre sizes, check your state or territory transport authority’s rules, and tell your insurer. Our article on compliant tyres and rims explains the risks.
When to keep OEM and when to change
| Your situation | Sensible choice |
|---|---|
| Daily commuting, mostly city and highway | Stay with the placard size |
| Wheels already changed by a previous owner | Return to the placard size, or confirm the current size is compliant |
| Towing or carrying heavy loads | Placard size or higher load capacity, never lower |
| Regular unsealed roads or off-road trips | Placard size in an all-terrain tyre, or a fitter-approved upgrade |
| Wanting a different look | A diameter-matched size, checked for load, clearance and rim width |
Our perspective
Most of the confusion we see comes from one thing: people treat tyre size as a three-number problem. They find 235/50R19, search for 235/50R19, and buy. The size is where you start, not where you finish.
We built Treadivo so the search starts from your car or tyre size, and the next step is a local fitter who can see your vehicle. Our tools, such as the size calculator and the AI tread check, help you understand what you have. They do not replace the placard and they do not replace a fitter’s eyes. If your car is modified, we would rather you ask a qualified fitter than lean on any online calculator, ours included.
Our advice is plain. Read the placard first. Stay with it unless you have a reason not to. If you do change, keep load and speed at or above original and let a fitter confirm clearance before you pay.
— Areesh
Buying the right size and getting it fitted
Once you know your size, you can browse tyres by category on our tyres page, including passenger, SUV and 4x4, light truck and more. Choose a local fitter near you, check out in about three minutes and receive a same-day fitting confirmation. If you are in Perth or Melbourne, see our pages for tyre fitting in Perth and tyre fitting in Melbourne. Not sure what fits? Our guide on what tyres fit your car covers rego lookup and ADR checks.
FAQ
What is the OEM tyre size?
It is the tyre size the vehicle manufacturer specified for your exact model and variant. You will find it on the placard, usually in the driver’s door jamb, and in the owner’s manual. If your car has had its wheels changed, the tyres currently on it may not be the OEM size.
What does OEM on tyres mean?
OEM stands for original equipment manufacturer. It means the tyre or size the carmaker fitted when the vehicle was built. Some tyres also carry a manufacturer-specific marking showing they were developed for a particular model, but the size itself is what most buyers need to match.
What is the legal tyre size in Australia?
There is no single legal size. The national rules require wheels and tyres suited to the vehicle’s size and load capacity, and tyre selection falls under the applicable ADR. For a modified car, check with a qualified fitter and your state or territory authority.
What is the 3% rule for tyre size?
It is a rule of thumb that overall tyre diameter should stay within about 3% of the original. It helps you screen sizes, but it is not a national legal test and does not approve a tyre on its own. Load capacity, speed rating, rim width and clearance still need to be right.
Sources
- Australian Design Rule 24, Tyre Selection (Australian Government)
- Australian Light Vehicle Standards Rules 2015 (consolidated 9 December 2022)