Chapter 5 — Section 4: Tyres and road wheels

Tyres represent the only point of contact between a vehicle and the road surface, making Section 4 of the MOT inspection manual one of the most critical for road safety. The inspection is conducted visually, with the vehicle raised on a lift to allow for full rotation of the wheels and internal sidewall examination. While the tester is not permitted to remove the wheels, they must assess the condition, suitability, and tread depth of all tyres fitted to the vehicle, including the spare if it is being used as a road wheel at the time of the test. A failure in this section often indicates a lack of routine maintenance, as most tyre defects are easily identifiable by the vehicle owner prior to the examination.

01Tread Depth and Measurement Standards

The legal minimum tread depth for most passenger vehicles (Class 4) is 1.6mm across the central three-quarters of the breadth of the tread, in a continuous band around the entire outer circumference. For motorcycles and larger goods vehicles, these requirements differ, but for the standard UK passenger car, the 1.6mm threshold is absolute. Testers use a calibrated tread depth gauge to take measurements. If any point within the central band falls below 1.6mm, the tyre fails.

• A 'Major' defect is recorded if the tread depth is below 1.6mm but above the threshold of immediate structural failure.
• A 'Dangerous' defect is triggered if the tread is worn to the extent that the cords or carcass are exposed or visible.
• Testers will often issue an 'Advisory' if the tread is between 1.6mm and 2.0mm, as the tyre is likely to fall below the legal limit before the next annual test.

The measurement must be taken across the tread pattern; it does not include wear on the extreme shoulders of the tyre unless that wear extends into the central 75% zone. However, if the shoulder wear is so severe that the textile cords are visible, the tyre will fail regardless of the depth in the centre.

02Sidewall Damage and Structural Integrity

The sidewall of a tyre is subject to immense pressure and heat. Inspecting this area requires a forensic approach to identify cuts, bulges, and perishing. Not all damage constitutes a failure. A cut in the rubber that does not reach the internal cords is typically recorded as an advisory unless it is deep enough to compromise the tyre's structure. However, if the cut is deep enough to expose the ply or cord, it is a Major failure.

• Bulges or bubbles indicate internal structural failure (delamination), usually caused by impact with a kerb or pothole. These are automatic 'Dangerous' failures.
• Perishing or 'crazing' caused by UV exposure and age is common on low-mileage vehicles. This is only a failure if the cracking is deep enough to expose the casing cords.
• Fouling is also checked; if a tyre rubs against any fixed part of the vehicle (such as a wheel arch liner or brake hose) at any point of its travel, it will fail.

03Tyre Mixing and Axle Consistency

The Road Vehicles (Construction and Use) Regulations 1986 dictate how different types of tyres can be mixed on a vehicle. For the purposes of the MOT, 'type' refers to the internal structure of the tyre—specifically whether it is radial-ply, cross-ply, or bias-belted. On a standard modern passenger car, all tyres are typically radial. Mixing a radial tyre with a cross-ply tyre on the same axle is a Major failure.

While it is legal to have different brands or tread patterns on the same axle, it is generally discouraged for handling stability. The MOT tester, however, is specifically looking for seasonal mixing. Since 2023, there has been closer scrutiny on the fitment of tyres. It remains legal to mix 'Summer' and 'Winter' tyres in the UK, provided they are of the same structure, though if a tester believes the combination makes the vehicle unstable, they may apply a failure under the 'General Condition' category. Additionally, tyres must be fitted according to the 'direction of rotation' markings on the sidewall; an asymmetric or directional tyre fitted backwards is a Major defect.

04Load Index and Speed Ratings

The load index and speed rating of a tyre must be appropriate for the vehicle's maximum gross weight and top speed. This is particularly relevant for vans and high-performance cars. The tester will check that the load capacity of each tyre is at least equal to half of the maximum axle weight as specified on the manufacturer’s VIN plate. Failure to meet this requirement is a Major defect.

• For vehicles first used on or after 1 January 1991, the speed rating must be appropriate for the vehicle’s maximum speed.
• Special 'Commercial' (C) rated tyres must be fitted to most light goods vehicles; fitting a standard passenger car tyre to a heavy van will result in an immediate failure due to insufficient load capacity.
• Tyres aged over 10 years are now a failure item on the front steered axles of heavy goods vehicles and buses, though this specific age-related failure does not currently apply to standard Class 4 passenger cars (though it is often advised).

05Road Wheels and Fixings

The inspection of the road wheels is as important as the tyres. The tester examines the wheels for cracks, significant distortion, and the security of the fixings. Steel wheels are prone to corrosion and denting, while alloy wheels are susceptible to cracking, often on the inner rim where it is difficult for the owner to see.

• A crack in a wheel is a Major failure; if the crack is deemed likely to cause the wheel to collapse, it is Dangerous.
• If a wheel bolt or nut is missing, loose, or the wrong type for the wheel (e.g., using a tapered nut on a radiused seat), the vehicle fails.
• Hubcaps or wheel trims that are damaged or insecure such that they might fly off or have sharp edges also constitute a failure.
• Heavily 'kerbed' alloys are usually only failed if the damage has compromised the seal of the tyre or resulted in a sharp edge likely to cause injury to pedestrians.

06Identifying 'Dangerous' Defects in Section 4 drowned

The distinction between 'Major' and 'Dangerous' is critical in Section 4. A Major defect allows the owner to drive the vehicle away for repair (provided the previous MOT is still valid), whereas a Dangerous defect means the vehicle should not be driven until the issue is rectified. In the context of tyres, 'Dangerous' is reserved for faults with an immediate risk to life. These include tyres with cords exposed, massive bulges, or wheels that are so loose they are at risk of detaching. The DVSA's stance is that any tyre which has reached the point of structural failure cannot be considered fit for even a short journey to a repair centre.

Tyre and wheel maintenance remains the most cost-effective way to ensure a vehicle passes its MOT. The majority of failures in Section 4 are preventable through basic monthly visual checks of tread depth and sidewall condition. As the only components governing grip and braking distance, the standards applied in this section represent the absolute minimum requirements for safe UK road use.