Every tyre has a size code printed on its sidewall — something like 205/55R16 91V. If you've ever wondered what those numbers and letters mean, this guide explains it all clearly so you can buy the right tyres with confidence.
Breaking Down the Tyre Size Code
Let's use 205/55R16 91V as our example — one of the most common tyre sizes on NZ roads, fitting cars like the Toyota Corolla, Honda Civic, and Mazda 3.
| Code | What It Means | Example |
|---|---|---|
| 205 | Section width in millimetres (measured from sidewall to sidewall) | 205 mm wide |
| 55 | Aspect ratio — sidewall height as a % of the width | 55% of 205mm = 113mm tall sidewall |
| R | Radial construction (standard on all modern passenger tyres) | Radial ply |
| 16 | Rim diameter in inches — must match your wheel exactly | 16-inch wheel |
| 91 | Load index — maximum weight each tyre can carry | 91 = 615 kg per tyre |
| V | Speed rating — maximum sustained speed | V = 240 km/h |
The Width (First Number)
The first number — 205 in our example — is the tyre's section width measured in millimetres. This is the distance from the outer sidewall to the inner sidewall when the tyre is mounted on its specified rim and inflated to the correct pressure.
Common NZ passenger car widths range from 175mm to 275mm. Wider tyres generally provide more grip and a sportier look, but they can increase fuel consumption and road noise. Narrower tyres tend to be more fuel-efficient and better in snow or mud.
The Aspect Ratio (Second Number)
The second number — 55 — is the aspect ratio, expressed as a percentage. It tells you how tall the sidewall is relative to the tyre's width.
In our example: 55% of 205mm = 112.75mm of sidewall height. A lower aspect ratio (like 35 or 40) means a shorter sidewall — common on performance cars. A higher ratio (like 65 or 70) means a taller sidewall — common on SUVs and 4WDs where ride comfort and off-road clearance matter.
The Construction Type (R)
The letter R stands for Radial construction — the standard for virtually all modern passenger, SUV, and light truck tyres. Radial tyres have ply cords that run perpendicular to the direction of travel, giving them better heat dissipation, fuel efficiency, and longevity compared to older bias-ply designs.
The Rim Diameter (Third Number)
The third number — 16 — is the diameter of the wheel rim in inches. This is the most critical measurement: you must always match this exactly. A 16-inch tyre will not fit on a 17-inch rim, and vice versa.
Modern NZ cars typically run on 15, 16, 17, 18, or 19-inch rims. Performance and luxury vehicles often go up to 20, 21, or even 22 inches.
Load Index and Speed Rating
After the rim diameter, you'll see two more characters: a number (load index) and a letter (speed rating). In our example, 91V:
The load index tells you the maximum weight each tyre can support. 91 means 615 kg per tyre — so the set of four can carry up to 2,460 kg. You should never fit a tyre with a lower load index than your vehicle's original specification.
The speed rating tells you the maximum sustained speed the tyre is designed for. V means 240 km/h. For most NZ drivers, H (210 km/h) or V (240 km/h) is more than adequate. See our full speed ratings guide for a complete breakdown.
Where to Find Your Tyre Size
There are four places to find your correct tyre size:
- On the tyre sidewall — look at the outer edge of your current tyres. The size is moulded into the rubber.
- Driver's door jamb sticker — open the driver's door and look for a white or yellow sticker on the door frame. It shows the recommended tyre size and inflation pressure.
- Owner's manual — the specifications section lists the factory-recommended tyre size.
- Fuel cap — some vehicles print tyre size and pressure on the inside of the fuel cap.
Can I Change My Tyre Size?
The rim diameter must always match exactly. However, small changes to width and aspect ratio are sometimes acceptable — for example, fitting a slightly wider tyre for better grip, or a lower-profile tyre for a sportier look.
However, changing the tyre size affects your speedometer reading, odometer accuracy, suspension clearance, and potentially your WOF compliance. Always consult a tyre specialist before deviating from your vehicle's original specification.
Not sure what size you need? Ask Charlie AI or use our vehicle search to find the exact right size for your car.