5.1 Tyre Tread, Damage, Winter Tyres & TPMS

Measure the correct grooves

Tyre inspection combines tread, construction, damage, repair, suitability, and electronic warning checks. Inspect every road tyre with the vehicle positioned so the complete circumference can be assessed.

Tread depth

For a normal tyre fitted with tread-depth indicators, at least 1.5 mm of tread must remain in all principal grooves that normally contain moulded tread-depth indicators, around the circumference. Principal grooves are the wide grooves containing the indicators, not fine sipes or decorative cuts.

The often-quoted “three-quarters of the tread width” method is not the default for every modern tyre. It applies where a tyre has no tread-depth indicators: the required depth must remain across the appropriate three-quarters of the width and around the circumference as the VIRM describes.

Do not rely only on a tread bar. Use a suitable gauge at the shallowest credible locations, avoiding the raised indicator itself. Uneven shoulder wear, flat spots, or local abrasion can produce a fail even when a single central reading is good.

Winter tyres

A winter tyre is identified using the VIRM markings and construction guidance. For classes MA, MB, MC, MD1, MD2, and NA, if one winter tyre is fitted, winter tyres must be fitted to all road wheels. Each requires at least 4 mm in all principal grooves that normally contain moulded tread-depth indicators, around the circumference. A pair on one axle is therefore not a compliant mixed seasonal set on those classes.

Condition and damage

Reject a tyre with a lump or bulge indicating structural separation; exposed or cut cords; tread or retread separation; a sharp object embedded in it; or significant perishing. A cut or crack is rejected when it is longer than 25 mm and reaches the cords. Do not rewrite that as “over 25 mm or reaches cords”; exposed/cut cords are already a separate reason and a superficial long mark may require careful classification.

A visible string-type puncture repair is rejected. Assess other repairs and damage using the page, including whether a repair has compromised the tyre structure. Check valve condition and any obvious inflation issue that affects safety.

TPMS and suitability

If the tyre-pressure monitoring system has an active warning indicating a fault or low pressure as specified by the VIRM, reject it. This is not limited to run-flat tyres. Confirm the warning is the tyre-pressure symbol rather than guessing from an unrelated cluster lamp.

The tread section does not replace the separate compatibility checks: size, carcass, load index, speed category, winter fitment, and permitted tyre type are covered in the next section.

Practical sequence

1. Identify tyre type and whether tread-depth indicators are present.
2. Rotate or move the vehicle as needed to inspect the entire circumference.
3. Measure the shallowest principal grooves using the correct method.
4. Look and feel for bulges, separation, cuts, cords, objects, perishing, and repairs.
5. Identify winter tyres and verify all-road-wheel fitment and 4 mm depth.
6. Observe the TPMS warning during the normal ignition/operation state.
7. Record tyre position, groove or damage location, measurement, and reason.

Scenario

A normal tyre measures 2.3 mm in two principal grooves containing moulded tread-depth indicators and 1.3 mm in the third such groove. It fails because every applicable indicator groove must meet 1.5 mm; an average above the limit is irrelevant. A winter tyre at 3.8 mm also fails its separate 4 mm rule.

Avoid measurement and damage errors

Stone holes and local tread features are not principal grooves. Find the tread-indicator symbols on the sidewall, identify the linked grooves, and sample around the full circumference. On a tyre without indicators, establish the normally road-contacting width before applying the three-quarter method. Do not average deep and shallow readings.

A bulge is structural evidence even if pressure is stable. Inspect the inner sidewall, which can hide impact damage or cords. If a screw remains embedded, the sharp-object reason applies; do not remove it during inspection and create rapid deflation. For perishing, distinguish minor superficial weathering from significant deterioration using the VIRM condition and visible structural evidence.

Final tyre checkpoint

Compare pressure-related shape and damage across the axle but do not use pressure correction to conceal a bulge or separation. Confirm that every tyre assessed is actually a road wheel; twin or unusual configurations may need the specific class page. On the checksheet, state measured depth and groove, tyre position, damage type and location. That evidence makes a 28-day recheck objective and repeatable.