Chapter 2 — Tyres: pressures, tread, age and the spare

Tyres represent the only point of contact between a vehicle and the road surface, with the total contact patch of a standard family hatchback being roughly equivalent to the surface area of four adult footprints. Neglecting these components increases braking distances and raises the risk of aquaplaning. Under UK law, specifically the Road Vehicles (Construction and Use) Regulations 1986, tyres must be fit for purpose and free from any defect which might cause damage to the road or danger to persons. Regular inspection is a matter of both mechanical maintenance and legal compliance.

01Understanding Sidewall Markings

The sidewall of a tyre contains every data point required to determine its suitability for your vehicle. A standard marking such as 205/55 R16 91V defines the physical and performance limits of the rubber. The first three-digit number is the width in millimetres. The second is the aspect ratio, indicating the sidewall height as a percentage of the width. 'R' denotes radial construction, followed by the wheel diameter in inches.

Critically, the final two indicators—the load index and speed rating—must meet or exceed the vehicle manufacturer’s original specifications. Fitting a tyre with a lower load index than specified is an MOT failure and may invalidate insurance. Additionally, look for the 'DOT' code, a four-digit sequence usually housed in a small oval. The first two digits represent the week of manufacture, and the last two represent the year. A code reading '1422' means the tyre was manufactured in the 14th week of 2022. Any tyre over ten years old is legally prohibited on the front axles of lorries and buses in the UK; while this law does not yet apply to passenger cars, professional consensus suggests replacing any tyre over six or seven years old regardless of tread depth.

02Tread Depth and the Legal Minimum

The UK legal minimum tread depth is 1.6mm across the central three-quarters of the breadth of the tread and around the entire outer circumference. However, performance degrades significantly before this limit is reached. Independent testing by organisations like MIRA shows that once tread depth drops below 3mm, the tyre's ability to displace water reduces exponentially. In wet conditions, the stopping distance from 50mph can increase by as much as 10 metres when comparing 3mm of tread to the legal 1.6mm minimum.

• Check tread depth using a calibrated gauge or the ‘20p test’ as a baseline (the outer rim of a 20p coin is roughly 2.7mm).
• Inspect for Tread Wear Indicators (TWIs), which are small raised bars in the grooves; when the tread is flush with these bars, the tyre is at the 1.6mm limit and requires immediate replacement.
• Look for 'sawtooth' wear or uneven smoothing on one edge, which typically indicates a wheel alignment (tracking) issue or worn suspension components.

03Pressure: The Cold Measurement Standard

Tyre pressures must be checked when 'cold'—defined as having been driven less than two miles or stationary for at least two hours. Driving heats the air inside the tyre, causing it to expand and providing an artificially high reading. Correct pressures are usually found on a sticker inside the driver’s door pillar, the fuel filler cap, or within the owner's manual. Note that most manufacturers provide two sets of figures: one for normal use and one for a fully laden vehicle.

Running tyres under-inflated causes the sidewalls to flex excessively, generating heat and increasing the risk of a high-speed blowout. It also increases rolling resistance, which negatively impacts fuel economy. Conversely, over-inflation reduces the contact patch, leading to a harsh ride and accelerated wear in the centre of the tread. Always ensure the valve dust caps are replaced, as these prevent grit from entering the valve core and causing slow leaks.

04The Dangers of Mixing Tyres on an Axle

The DVSA MOT inspection manual is explicit regarding tyre compatibility: tyres on the same axle must be of the same structure. This means you cannot mix a radial tyre with a cross-ply tyre. While it is legal to have different brands or tread patterns on the same axle, it is strongly advised against. Different manufacturers use varied rubber compounds and tread designs, leading to disparate grip levels and water displacement rates. This can cause unpredictable handling during emergency braking or cornering.

• Tyres must be of the same size and load/speed rating across an axle.
• Directional tyres must be fitted so the arrow on the sidewall points in the direction of forward rotation. Fitting these backwards is an MOT failure.
• If replacing only two tyres, the newer rubber should ideally be fitted to the rear axle to provide better stability and prevent oversteer in wet conditions, regardless of whether the car is front- or rear-wheel drive.

05The Spare Tyre and the Four-Year Rule

Many modern vehicles are sold with 'spacer-saver' temporary spares or inflation kits rather than full-sized replacements. If your vehicle has a space-saver, it will often carry a strict speed limit, typically 50mph (80km/h), and is designed only to get you to the nearest tyre bay. These must never be used for prolonged periods or at motorway speeds.

Crucially, spare tyres suffer from atmospheric degradation even if they have never touched the road. Experts generally recommend a 'four-year rule' for spares: inspect them every year, and consider replacement after four years of storage, even if the tread is as-new. Rubber hardens and develops micro-cracks over time, a process known as 'dry rot'. An old spare may appear functional but could disintegrate under the stresses of heat and load when finally put into service. If your car uses a tyre sealant kit, check the expiry date on the canister; these chemicals have a finite shelf life and will fail to seal a puncture once expired.

06Summary of Pre-Drive Checks

Maintaining tyres involves more than checking for a flat. Verification of the DOT manufacture code, ensuring pressures match the vehicle load, and confirming that tread depth remains safely above the 1.6mm legal floor are essential tasks. Regular observation prevents the use of compromised rubber and ensures the vehicle remains compliant with UK road traffic laws.