Find your tyre size — and read everything else on the sidewall

Reading the code: 205/55R16 91V

Almost every car, van and SUV tyre sold in New Zealand uses the metric format. It is one line of raised lettering on the outboard sidewall, and it is usually the largest text on that face of the tyre. Six parts, read left to right.

Want to see those six parts on an actual sidewall? There is a close-up further down, in load index — a 265/60R18 114T with every character readable.

The one that catches people out is the profile. It is a percentage, not a measurement, so the same "55" means a different sidewall height on every width. 205/55R16 has a 113 mm sidewall; 255/55R18 has a 140 mm sidewall from the same profile number. That is why a 55-profile tyre is not automatically a comfortable one — low profile is about the millimetres, not the percentage.

Imperial and flotation sizes: 31×10.5R15 109Q

Common on utes and 4WDs — Rangers, Hiluxes, Navaras, Defenders — and on anything running an all-terrain or mud-terrain tyre. The format is easier than metric once you know the trick: the first number is the overall height in inches, not the width.

How it differs from metric

• The first number is diameter, in inches. A 31 is 31 inches tall, near enough 787 mm. Metric makes you calculate that; imperial hands it to you.
• The second number is width, in inches. 10.5 inches is about 267 mm.
• There is no aspect ratio — it is not needed, because the height is already stated. Sidewall height = (overall diameter − rim diameter) ÷ 2.
• Comparing sizes is trivial. Going from a 31 to a 33 is two inches taller, full stop. That is exactly why the format survives on off-road tyres.

Where to find your tyre size

There are three places to look, and they do not always agree. Two of them tell you what is fitted; only one tells you what the vehicle was designed for.

1. The sidewall of the tyres you haveLook at the outboard face of any tyre. The size is moulded in raised letters, usually starting with three digits. Write down the whole line including the load index and speed rating at the end — the last two characters are the ones people forget, and they are the ones that decide what you may legally fit.
2. The placard on the vehicleA sticker or plate listing the sizes and pressures the manufacturer specified. On New Zealand vehicles it is nearly always in the driver door jamb; also try the fuel flap, the glovebox (inside the door or on the back wall) and, on some European cars, under the bonnet.
3. The owner manualLists every approved size for the model, which matters because many cars were sold with two or three wheel options. It also gives the pressures for a loaded vehicle, which the placard sometimes leaves out.

Why the manufacturer size is the one to keep

The size on the placard is not a suggestion. It is the size the brakes, the speedometer, the stability control and the wheel arches were all developed around. Changing it changes:

• Speedometer and odometer accuracy — a bigger rolling circumference makes the speedo under-read, so you drive faster than it says.
• WOF compliance — the overall diameter must stay within 5% of standard without certification.
• Load and speed capability — a lower load index or speed rating than the vehicle needs is a fail.
• Clearance — a wider or taller tyre can foul the arch, the strut or the suspension arms at full lock or full compression.
• Handling and braking — every published figure for the car was measured on the standard size.

Size calculator: what your numbers actually measure

Type your size in and see what it comes to in millimetres. Change one number at a time to see what a plus-size or a profile change really does to the diameter — anything more than 5% away from standard is a problem for your speedometer and your WOF.

Width (mm)

Profile %

Rim (inches)

Load index

205mm 632mm

Head on — section width against overall diameter, drawn to scale against each other

113mm

From the side — the arrow is the sidewall itself, measured from the tread down to the rim. That height is what the profile number sets.

Overall diameter632mm

Sidewall height113mm

Circumference1985mm

Load capacity615kg

Revolutions per km504

Load capacity is per tyre, at the pressure moulded on the sidewall. Revolutions per kilometre is what your speedometer is calibrated against — change it and the speedo reads wrong by the same percentage.

Older and odd size formats

Four formats turn up often enough in New Zealand to be worth knowing: numeric truck sizes, American alpha-numeric, metric sizes with the profile left off, and light-truck sizes carrying two load indices. If your size will not go into an online size picker, it is almost certainly one of these.

Numeric truck sizes: 7.50R16

Used on older light trucks, commercial vehicles and some trailers. The coded number is not the section width, which is the mistake nearly every conversion chart makes. A 7.50R16 codes to 190.5 mm if you simply multiply by 25.4, but its actual design section width is 220 mm — 15% wider than the code implies.

There is also no aspect ratio you can recover from the code. These sizes came from a historical sequence of nominal widths rather than a height-to-width formula, so the dimensions live in standards tables (UN Regulation No. 54), not in a calculation. Work the ratios backwards and you get 95.8%, 92.0%, 91.3% and 93.9% for four sizes in the same family — no single factor fits.

Dimensions from UN R54 design data. The effective profile is worked backwards from the published figures — it is not printed on the tyre and should never be quoted as if it were. Our size calculator holds 13 of these sizes and declines any it does not have rather than guessing.

American alpha-numeric: GR78-15

Pre-1980s American cars. The letter is a load range, the two digits are the aspect ratio and the last number is the rim.

No profile number: 185R14

When the aspect ratio is missing, it is not missing by accident — it is the convention for an 82% profile. Most charts online round that to 80% because it is a tidier number, and every size then comes out about 1.3% small.

Two load indices: LT265/70R17 121/118S

Which one applies to you? If your vehicle has one wheel at each corner — a Ranger, Hilux, Navara, Colorado, Amarok — use the first number. The lower second figure exists because paired tyres share load unevenly and run hotter.

Motorcycle sizes

Motorcycles use their own systems: metric like 120/70ZR17, alpha-numeric like MT90-16, and numeric like 4.00-18 on classics. They carry different load and speed conventions and a different set of WOF checks, so we have deliberately left them out of this guide rather than half-cover them. Talk to a motorcycle specialist.

Load index: the number before the speed letter

The load index is a code, not a weight. 91 means 615 kg per tyre, at the pressure moulded on the sidewall. Four tyres at 91 give a tyre capacity of 2,460 kg — which has to be more than the vehicle plus everyone and everything in it, with room for the fact that load never sits evenly across the axles.

The indices you will actually meet

The full chart from 50 to 160 is on our load rating chart, and the load rating guide works through upgrades and towing.

What the WOF rule actually says

People repeat that both tyres on an axle must have the identical load index. That is not what the manual says. NZTA's reason for rejection is that the tyres on an axle of a light vehicle do not meet at least one of the following: the ply ratings are the same, or the load indices differ by no more than 2.

Worth the extra money: a real example

Tread pattern type — and why it limits what you may fit

Pattern type is not decoration and it is not just a comfort choice. It is one of the three things that must match across an axle, so it decides which tyres can be fitted beside the ones you already have. There are three types, and you can tell them apart in about five seconds.

The quickest test is the centre-line method: draw an imaginary line down the middle of the tread and compare the two halves. Three of our own tyres, each with that line drawn on:

Which one is best for New Zealand is a longer conversation — pattern families, sipes, void ratio, noise, gravel chipping and what actually wears fastest. It is all in the tread pattern guide, with 54 questions answered and photographs of each family.

Is it a run-flat, a reinforced tyre, or something else special?

Two or three letters on the sidewall can change what the tyre needs from you and what you are allowed to fit beside it. Check for these before you shop, because a like-for-like replacement is not always the obvious one.

XL, reinforced and light load

Of the 1,601 tyres we have inspected and recorded, 27.6% carried an XL or reinforced marking — more than one in four cars on the road needs the higher pressure and, in our experience, is not running it.

Run-flat codes

Construction codes

Snowflake and M+S markings are a different question again — they are about the compound and the pattern rather than the casing. Seasons and snow markings covers them.

Tyre age: what the WOF really says about it

Rubber hardens and cracks whether or not the car is driven, so a tyre can have 7 mm of tread and still be finished. The date it was built is moulded into the sidewall as the last four digits of the DOT code: week, then year.

UTQG: treadwear, traction and temperature

Three grades, moulded together on the sidewall of most tyres built for the American market. They are the closest thing a sidewall gives you to a durability and grip rating — with one important limit, below.

UTQG is a United States grading scheme, so it appears on tyres built for that market and is absent from plenty of others. Where it is present it gives you three things: treadwear (relative wear rate, higher lasts longer), traction (AA, A, B or C — wet straight-line braking on a test surface) and temperature (A, B or C — heat resistance at speed). Treadwear is graded by each manufacturer against its own control tyre, so it compares models within a brand far better than it compares brands. What the grades actually predict, measured against our own stock, is written up in the low profile tyre guide.

All-season, M+S, winter or summer — which have you got?

Rubber is a compromise between grip when it is warm and grip when it is cold, and no compound wins at both. The sidewall tells you which compromise the maker chose, but only one of the markings is backed by a test.

Tread depth: 1.5 mm is the law, 3 mm is the advice

The rule, quoted: a tyre other than a winter tyre, on a vehicle capable of exceeding 30 km/h, must have a tread pattern depth of at least 1.5 mm around the whole circumference, in the principal grooves, excluding any tie-bar or tread-depth indicator strip. Winter tyres need 4 mm.

Being legal and being safe are different things. Wet grip falls away quickly below about 4 mm, and by 1.5 mm a car needs metres more to stop in the rain than the same car on 3 mm. We tell customers to replace at 3 mm, and we say the same thing when it costs us a sale.

New depth is not the same on every tyre

This trips up more people than the legal limit does. If a passenger tyre and a mud-terrain both read 6 mm, the passenger tyre is barely worn and the mud-terrain is more than half gone.

Photographs of real tyres at every depth from new to illegal, a braking-distance calculator and the full wear-pattern gallery are in the tread depth guide.

Plus-sizing: bigger wheels without breaking anything

Plus-sizing means going up a rim size while keeping the overall diameter the same, by dropping the profile to give back what the bigger wheel took. Do it properly and the speedometer stays honest, the arch clearance stays the same and the WOF is a non-event.

What else to check before committing

• Load index. Lower-profile tyres in the same width sometimes carry a lower index. It still has to meet the placard figure.
• Rim width. Every tyre width has a range of rim widths it is approved on. Too narrow or too wide a rim distorts the profile and the handling with it.
• Clearance at full lock and full compression. The tyre that clears in the driveway is not necessarily the one that clears over a judder bar with four people aboard.
• Ride quality. Less sidewall means less to absorb a pothole, and New Zealand supplies plenty. Our low profile guide covers what that costs you in comfort and wheel damage.

The rest of the sidewall: prefixes, approvals and certification

Everything above is something you act on. What is left is mostly identification — who the tyre was built for, and which standards it was built to.

Letters before the size

Manufacturer approval codes

An approval code means the tyre was tuned for that car weight and suspension. You are not obliged to buy one. A non-approved tyre of the same size, load index and speed rating is legal and usually cheaper. On a car whose ride and steering were signed off around a specific tyre — most obviously Porsche, AMG and M cars — the approved version is worth the money; on a family SUV it rarely is.

Common questions about tyre sizes

The three most-asked questions in each area, with a link to the guide that answers the rest.

Tools that do the arithmetic for you

Everything on this page is free and needs no login. If you already know your size, the calculator and the axle checker will answer most buying questions in under a minute.

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