Tyre Size Chart: How to Read, Find and Compare Sizes
A tyre size such as 205/55R16 91V packs six facts into one code: width in millimetres, sidewall height as a percentage of that width, construction, rim diameter in inches, load index and speed rating. Below you’ll find a chart of common Australian sizes with their overall diameters, a plain-English decoder, the places your car’s size is printed, and the limits on moving away from the factory size.
TL;DR
- Read the code left to right: width (mm), aspect ratio (%), R for radial, rim diameter (inches), then load index and speed rating.
- Your size is on the tyre sidewall and on the placard in the driver’s door jamb. Check all four tyres, as front and rear can differ.
- Compare sizes by overall diameter, not by width alone. A change of more than a few millimetres can upset your speedometer and may not be legal.
- Never go below the load index and speed rating your manufacturer specifies.
- Not sure? Use our tyre size calculator to compare two sizes before you order.
Table of Contents
- How to read a tyre size
- Load index and speed rating
- Tyre size chart: common sizes and their overall diameters
- How to find your tyre size
- Comparing two tyre sizes
- Can you fit a different size in Australia?
- Our view: size is the easy part
- Ready to buy? Order your size and book the fitting
- FAQ
- Sources
How to read a tyre size
Take 205/55R16 91V, a very common size on Australian small and medium cars. Here is each part:
| Part | Example | What it tells you |
|---|---|---|
| Section width | 205 | Width in millimetres, sidewall to sidewall |
| Aspect ratio | 55 | Sidewall height as a percentage of the width |
| Construction | R | Radial |
| Rim diameter | 16 | Wheel size in inches |
| Load index | 91 | Maximum load one tyre can carry |
| Speed rating | V | Maximum speed at that load |
This matches how Kumho Australia breaks down 185/75R14 89T: width in millimetres, aspect ratio, radial construction, rim diameter in inches, then load index and speed symbol. CHOICE adds that radial construction is the most common type on passenger cars and is more durable than older bias-ply tyres.
The aspect ratio trips people up. A 55 doesn’t mean 55 mm. It means the sidewall is 55% of the width. On a 205 tyre that’s about 113 mm. A lower number means a shorter, stiffer sidewall, usually with a sportier look and a firmer ride.
If you want a worked example of one size, our guide to what 205/55R16 means goes through it line by line.
Metric and imperial markings
Some older or 4x4-style tyres use inch-based sizes, such as 31x10.50R15, where 31 is the overall diameter and 10.50 the width, both in inches. Queensland’s transport department notes that tyres carry either imperial or metric coding, and that you convert inches to millimetres by multiplying by 25.4. So a 275 tyre is about 10.8 inches wide, and a 10.50-inch tyre is about 267 mm wide.
Load index and speed rating
The two characters at the end of the code matter as much as the size.
Load index is a code for the maximum load a single tyre can carry. Speed rating is a code for the maximum speed at which it can carry that load, as Michelin Australia explains. Queensland’s guidance says the approved indices and speed ratings for passenger car tyres sit in ADR 23/02. CHOICE notes that new tyres must meet at least the load rating your car’s manufacturer specifies.
Two practical points:
- A load index like 149/146 on a light truck or commercial tyre is a pair: the first figure is for single fitment, the second for dual (twin) fitment.
- The speed letter is a capability rating, not a target. Drive’s example has T corresponding to 190 km/h. Always fit the same or higher than the original.
We’ve written deeper guides on both: what a load index such as 91 means in kilograms and why you should never fit a lower speed rating.
Tyre size chart: common sizes and their overall diameters
Overall diameter is what decides whether a tyre fits and whether your speedometer stays accurate. The formula is simple: rim diameter in mm (inches x 25.4) plus two sidewalls (width x aspect ratio / 100, doubled). We’ve worked these out for sizes Australian drivers commonly search for. Treat them as calculated approximations: the real tyre’s published diameter varies slightly by brand and design.
| Size | Sidewall height | Rim | Overall diameter (approx.) | Typical use |
|---|---|---|---|---|
| 185/65R15 | 120 mm | 15 in | 622 mm | Small hatchbacks |
| 195/65R15 | 127 mm | 15 in | 635 mm | Small sedans and hatchbacks |
| 205/55R16 | 113 mm | 16 in | 632 mm | Small to medium cars |
| 215/60R16 | 129 mm | 16 in | 664 mm | Medium sedans, small SUVs |
| 225/45R17 | 101 mm | 17 in | 634 mm | Sportier hatchbacks and sedans |
| 245/40R18 | 98 mm | 18 in | 653 mm | Performance sedans |
| 225/65R17 | 146 mm | 17 in | 724 mm | Mid-size SUVs |
| 235/60R18 | 141 mm | 18 in | 739 mm | Larger SUVs |
| 265/65R17 | 172 mm | 17 in | 776 mm | 4x4 utes and wagons |
| 265/60R18 | 159 mm | 18 in | 775 mm | Large SUVs and 4x4s |
| 265/70R17 | 186 mm | 17 in | 803 mm | 4x4 and light truck |
Notice how 205/55R16 and 225/45R17 land within 2 mm of each other in diameter. That’s the logic behind “plus sizing”: go up a rim size, widen the tyre and shrink the sidewall so the overall diameter barely changes. Our guide to plus sizing tyres and four Australian fitment checks covers this in depth.
Want every size we list rather than a handful? Browse the tyre sizes directory.
Is a 285 bigger than a 275?
Yes, by 10 mm of width, if the aspect ratio and rim are equal. But the real change in diameter depends on the aspect ratio too. A 285/60R18 is wider and also about 8 mm taller overall than a 275/60R18 (each sidewall is 6 mm taller), which is why you compare diameters, not just the first number.
What is the 7-7 rule?
This phrase turns up in searches but isn’t about size. SlashGear explains it as a winter-tyre rule of thumb tied to outside temperature, which has little relevance to tyre dimensions in most of Australia. Ignore it when choosing a size.
How to find your tyre size
You have four reliable places to look.
- The tyre sidewall. The size is moulded into the rubber, for example 205 55 R16 91V, as RACV’s tyre guide shows.
- The tyre placard. Usually a sticker in the driver’s door jamb. Kumho says it can also be in the glovebox or fuel-filler area. Tasmania Transport’s guidance, as summarised by RAA, says the placard lists recommended wheel and tyre combinations, pressure, load capacity and speed rating.
- The owner’s manual or vehicle documents.
- Our directory, if you only know the car. Search by model on Treadivo and you’ll see the fitments listed.
Check all four tyres. Kumho warns that front and rear sizes may differ, which is common on performance cars. If the tyres on your car aren’t the original size, the placard is the better guide to what the manufacturer intended. Our article on finding your car’s original (OEM) tyre size explains how to tell the difference.
While you’re looking at the sidewall, check the wear too. Guidance on minimum tyre tread depth is a good reminder of when worn tyres need replacing, whatever size you buy.
Comparing two tyre sizes
When you compare sizes, you’re really checking three things: overall diameter, width and aspect ratio. Continental explains that compatibility comes down to outer dimensions, including diameter or rolling circumference and width, and that online tools can show the differences between two sizes.
A worked example. You have 205/55R16 (632 mm) and you’re considering 215/55R16:
- Width: 215 mm vs 205 mm, so 10 mm wider.
- Sidewall: 118 mm vs 113 mm, so 5 mm taller each side.
- Overall diameter: about 643 mm vs 632 mm, so about 11 mm larger, or roughly 1.7% bigger.
That’s a small change on paper, but it moves your speedometer reading and can matter for clearance. Run your own numbers in the tyre size calculator, which shows two sizes side by side.
Wheel width matters too
A tyre must suit the rim it’s mounted on, so a wider tyre isn’t always a free swap. Our guide to how wheel and tyre sizes match explains how rim width limits the tyre widths you can safely fit.
Can you fit a different size in Australia?
Often yes, within limits. The rules are state-based, and the clearest published example is Queensland’s Section LS guidance. Check your own state’s rules or ask a fitter before you change anything.
What Queensland’s Section LS guidance says:
- A passenger-car tyre’s overall diameter must be no more than 15 mm larger or 26 mm smaller than a manufacturer-designated tyre.
- Specified off-road 4WDs and certain goods vehicles may go up to 50 mm larger or 26 mm smaller, subject to other requirements.
- Speedometer accuracy must be maintained.
- Front-to-rear aspect ratio variation is limited to 10.
- Rims on each axle must share diameter, offset, width and mounting configuration.
- Passenger-car tyres can’t be more than 30% wider than the manufacturer’s widest optional tyre.
Apply that to our example. If your car came with 205/55R16 (about 632 mm), the diameter window under that guidance is roughly 606 mm to 647 mm. A 215/55R16 at about 643 mm sits inside it. A 225/55R16 at about 654 mm doesn’t.
CHOICE’s advice is simple: stick to the exact size the car comes with, or upgrade to a specific combination. On most cars the rims can go one or two inches bigger, with different widths and aspect ratios. RAA adds that tyres must be the size and construction the manufacturer specifies, and that tyres on the same axle should always match.
Our NCOP11 tyre size calculation guide walks through these limits step by step.
A note on UTQG
You may see UTQG treadwear, traction and temperature figures on imported tyres. They’re a US grading scheme and not part of the size code. We explain how to read UTQG ratings in an Australian context.
Our view: size is the easy part
We built Treadivo because most people don’t struggle to read 205/55R16. They struggle to be sure it’s the right tyre for their car, at a price that doesn’t change at the counter, fitted by someone local.
Our advice, from watching how people shop for tyres:
- Start with the placard, not a chart. A chart tells you what exists. The placard tells you what your manufacturer approved.
- Treat load index and speed rating as minimums. A tyre that matches on size but falls short on either isn’t a match.
- Change size on purpose. If you want a different look, compare overall diameter first, and check the state rules and your insurer’s view before you commit.
- Look before you buy. Our augmented reality feature lets you see how tyres look on your car, which takes some of the guesswork out of plus sizing.
A chart is a starting point. A fitment you can confirm is the finish line.
— Areesh
Ready to buy? Order your size and book the fitting
Once you have your size, search by model or tyre size on Treadivo, choose from local fitters near you and check out in about three minutes, with same-day fitting confirmation and no hidden fees. We list passenger, SUV and 4x4, light truck, truck, commercial, highway, mud terrain and motorcycle tyres. If you’re in Melbourne or Perth, see tyre fitting in Melbourne or tyre fitting in Perth.
FAQ
How do I decode a tyre size?
Read left to right: width in millimetres, aspect ratio as a percentage of width, a letter for construction (R is radial), rim diameter in inches, then load index and speed rating. In 205/55R16 91V, the tyre is 205 mm wide, has a 113 mm sidewall, fits a 16-inch rim and has a load index of 91 and a speed rating of V.
Where do I find my tyre size?
Check the sidewall of the tyre and the placard in the driver’s door jamb. The owner’s manual also lists it. Check all four tyres, as front and rear sizes can differ.
What size is a 275 tyre in inches?
The 275 is the width in millimetres, which is about 10.8 inches (275 divided by 25.4). The overall diameter depends on the aspect ratio and rim size. A 275/60R18, for example, is about 775 mm, or roughly 30.5 inches, tall.
Can I fit a different tyre size to my car?
Sometimes, within limits. Queensland’s guidance allows passenger-car tyres up to 15 mm larger or 26 mm smaller in overall diameter than a manufacturer-designated size, while keeping the speedometer accurate. Other states may differ, so check your local rules or ask a fitter first.
Sources
- How to read a tyre - CHOICE
- Tyre information - Queensland Transport and Main Roads
- How to read tyre size - Kumho Tyre
- What do the numbers on the side of tyres mean? - Drive
- Tyre markings explained - Michelin Australia
- Car tyre buying guide - RACV
- Tyres - RAA help centre
- Tire size guide - Continental
- Section LS Suspension and Steering - Queensland Transport and Main Roads
- What Is The 7-7 Rule For Tires? - SlashGear