How to Read Tyre Sizes — What Every NZ Driver Needs to Know

Understanding the numbers and letters on your tyre sidewall is simpler than it looks. Every combination tells you the tyre’s exact dimensions, its speed capability, and its load capacity — all critical when buying replacement tyres in New Zealand.

The Standard Tyre Size Format

Most tyres in NZ use this format:

225 / 45 R 17 91 W

Each number and letter means something specific:

Breaking Down the Numbers

225 — Tyre Width (millimetres)

The first number is the tyre’s width across the tread, measured in millimetres. A wider tyre has a larger contact patch with the road — generally improving grip but also increasing rolling resistance and fuel consumption.

Common widths in NZ: 175 (small cars) → 225 (family sedans) → 275 (sports cars) → 315 (performance SUVs)

45 — Aspect Ratio (%)

The second number is the aspect ratio — the height of the tyre’s sidewall as a percentage of its width.

• 45 means the sidewall height is 45% of 225mm = 101mm
• A lower aspect ratio (e.g. 35) means a shorter sidewall — sportier look and handling but firmer ride
• A higher aspect ratio (e.g. 65) means a taller sidewall — more comfort, more suited to rough roads

NZ road note: New Zealand’s chipseal roads are harder on tyres than smooth European asphalt. Many NZ drivers opt for slightly higher aspect ratios than they might in Europe for better impact resistance.

R — Radial Construction

The letter R indicates the tyre is a radial construction — the standard for all modern car tyres. The ply cords run across the tyre from bead to bead at 90 degrees to the direction of travel. Virtually every tyre sold in NZ is a radial.

17 — Rim Diameter (inches)

The number after R is the rim size the tyre fits, measured in inches. This must exactly match your wheel size. A 17″ tyre will not fit on a 16″ or 18″ rim.

Common NZ rim sizes: 14″–15″ (economy cars) → 16″–18″ (family vehicles) → 18″–22″ (SUVs and performance cars)

91 — Load Index

The load index tells you the maximum weight the tyre can safely carry when properly inflated. This is a code number — not the actual weight in kilograms.

Common load index values:

NZ WoF requirement: Your replacement tyre’s load index must equal or exceed the vehicle manufacturer’s specification. Fitting a lower load index tyre is a WoF failure and is unsafe.

W — Speed Rating

The speed rating is the final letter — it indicates the maximum safe continuous speed the tyre is designed for.

NZ WoF requirement: Your replacement tyre’s speed rating must equal or exceed the original vehicle specification. Fitting a lower speed rating tyre is a WoF failure.

Practical note for NZ: New Zealand’s maximum open road speed is 100 km/h. Any tyre rated H (210 km/h) or above is more than adequate for legal road use. Matching the manufacturer’s specification is the relevant requirement, not the actual speed driven.

Other Markings You’ll See on NZ Tyres

XL (Extra Load)

Tyres marked XL or REINF can carry more weight than a standard tyre of the same size when inflated to their maximum pressure. Common on SUVs and performance cars that need additional load capacity within the same tyre size.

Example: 225/45 R17 94W XL can carry 670 kg vs 615 kg for the standard 91W version.

M+S (Mud and Snow)

This marking indicates the tyre has a tread pattern designed to perform in mud and light snow conditions. Note: M+S does NOT mean the tyre is a proper winter tyre. The Three-Peak Mountain Snowflake symbol (3PMSF — three overlapping mountain peaks with a snowflake inside) is the relevant certification for genuine winter/snow performance. Many tyres carry M+S without having the 3PMSF certification.

3PMSF (Three-Peak Mountain Snowflake)

This symbol certifies the tyre has passed an independent snow traction test. For South Island drivers in alpine areas, this is the meaningful winter tyre indicator — not M+S alone.

DOT Code and Tyre Age

Every tyre has a DOT (Department of Transportation) code on the sidewall. The last four digits indicate the manufacturing date:

Example: DOT XXXX 2318 = Week 23 of 2018 = manufactured May/June 2018

Why this matters in NZ: NZ’s high UV intensity accelerates tyre rubber ageing. Industry guidance recommends replacing tyres after 7–10 years, regardless of tread depth. A tyre with good tread but manufactured in 2015 is 10+ years old and should be replaced even if it looks fine.

How to Find Your Correct Tyre Size

There are three reliable sources for your vehicle’s correct tyre size:

1. Driver’s door jamb sticker — a label inside the driver’s door lists the OEM tyre size and recommended pressures
2. Owner’s manual — usually in the specifications section
3. Current tyres — read the sidewall of your existing tyre (only if you know the original size hasn’t been changed)

Do not assume because a tyre physically fits your rim that it is the correct specification. Incorrect sizes can affect speedometer accuracy, WoF compliance, and vehicle handling.

NZ-Specific Considerations

Replacing One or Two Tyres vs All Four

For WoF compliance, tyres on the same axle must be the same size. If replacing two tyres (front or rear axle), both must match in size and tread pattern type. It is recommended to replace all four tyres together on AWD and 4WD vehicles, where mismatched wear levels can damage differentials.

Upgrading Tyre Size

Some NZ drivers “plus-size” — fitting larger diameter or wider tyres than OEM. This is legal if:

• The tyre physically clears all suspension and bodywork throughout full steering lock
• Speedometer accuracy is maintained within legal tolerances
• The new size is equal or greater in load and speed rating
• Your Warrant of Fitness inspector approves the modification

Quick Reference Cheat Sheet

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225 / 45 R17 91W XL

+– Tread Width (225 mm)

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Sources

1. NZTA Vehicle Inspection Requirements — vehicleinspection.nzta.govt.nz — accessed 2026-06-01
2. ETRTO tyre size standards — etrto.org — accessed 2026-06-01
3. Tyre load and speed rating tables — tyrereviews.com — accessed 2026-06-01

Related Pages

• NZ WoF Tyre Requirements — legal requirements
• Tyre Pressure Guide NZ — correct pressures for your vehicle
• Tyre Age Guide — when to replace on age