5.2 Tyre Size, Carcass, Load/Speed Categories & Modifications

Compatibility is an axle and vehicle question

Read every sidewall before deciding that four tyres “look the same.” The VIRM uses labelled size, carcass, tread pattern, load/speed category, purpose, and modification effects.

Same-axle rules

Tyres on the same axle must have the same:

• size designation;
• carcass type; and
• tread pattern.

For the applicable MA, MB, MD1, and NA vehicles first registered or re-registered in New Zealand on or after 1 October 2002, all road tyres must also have the same carcass type, subject to the stated slow-vehicle and age exceptions. This rule is clearer and safer than the outdated shortcut about whether radial tyres may be placed on one particular axle.

Ply ratings on the same axle must be the same, or load indices may differ by no more than two. The fitted tyre must still have adequate load capacity for the vehicle and position; a two-index difference does not excuse an under-rated tyre.

Speed and permitted tyre types

The speed category must be at least the speed limit applicable to the vehicle, or the vehicle’s maximum speed where that is lower. It is not automatically required to match a theoretical original design speed that cannot legally or physically be reached under the applicable rule.

Reject a space-saver, non-pneumatic tyre, studded tyre, or tyre marked unsuitable for highway use when fitted as a normal road wheel in circumstances prohibited by the page. A temporary spare carried in its stowage position is not a road wheel; once fitted for the inspection, its restricted-use status matters.

Winter tyres must be fitted to all road wheels on the listed light classes, as covered in section 5.1.

Size changes and projection

Compare the outer circumference with original equipment. An increase of more than 5% is outside the uncertified allowance unless the modification has the proof required by the VIRM. Also inspect clearance through steering lock and suspension travel, load/speed suitability, and whether tread projects beyond permitted body coverage.

Wheel spacers and adaptors are not accepted as a casual method of correcting offset. Apply the modification table and require LVV or other accepted evidence where the fitting is not excluded. A tyre or wheel modification can affect steering, suspension, guards, speedometer accuracy, and braking; open those pages as necessary.

Sidewall reading method

A marking such as 225/45R17 94W provides nominal width, aspect ratio, radial carcass, rim diameter, load index, and speed category. Use an authoritative conversion source where necessary rather than estimating from appearance. Inspect both sides of the axle because the inner sidewall may reveal a different size or winter marking.

Inspection sequence

1. Record all four size designations and carcass markings.
2. Compare tread patterns on each axle.
3. Compare ply ratings or load indices and verify capacity.
4. Check speed category against the applicable requirement.
5. Identify winter, space-saver, non-highway, studded, or non-pneumatic tyres.
6. Compare modified circumference with OE and inspect clearance/projection.
7. Verify certification evidence when a modification exceeds a threshold.

Scenario

Two rear tyres are the same 205/55R16 size and radial construction, but have visibly different tread patterns. They fail the same-axle compatibility rule even if depth and load indices are acceptable. Conversely, different brands can pass when the required construction, size, pattern, and ratings comply.

Load and modification cross-check

Convert load indices with an authoritative table and consider the applicable axle loading; the index printed on a tyre is not a kilogram value. A pair can satisfy the “difference no more than two” rule yet still be under-capacity, so complete both checks. Likewise, a speed category letter must be interpreted, not alphabetically guessed.

When calculating circumference change, compare like-for-like nominal dimensions or measured data accepted by the VIRM and show the percentage: (new circumference minus OE circumference) divided by OE circumference, multiplied by 100. Then inspect physical clearance under steering and suspension travel. A change below 5% can still fail for contact, projection, load, or incompatible construction.

Final compatibility checkpoint

Re-read complete sidewall markings after rotating the wheel enough to find suffixes, reinforced/load-range information, winter symbols and “temporary use” wording. A nominally matching size can conceal different construction or purpose. If a marking is unreadable, use the VIRM’s evidence path rather than assuming the tyre is adequate. Finish by confirming that axle conclusions remain valid after any spare or temporary wheel is identified.