How to Read Tyre Size Numbers
A typical UK tyre marking such as 205/55 R16 91V contains all the key specifications. The first number, 205, is the tyre width in millimetres measured across the tread face. The second number, 55, is the aspect ratio, meaning the sidewall height is 55 per cent of the width. The letter R indicates radial construction, which is standard on all modern car tyres. The number 16 is the rim diameter in inches. The load index, 91, indicates the maximum weight each tyre can carry, and the speed rating, V, specifies the maximum speed the tyre is designed for.
When changing tyre sizes, the overall rolling diameter should remain within 3 per cent of the original to avoid affecting speedometer accuracy, ABS operation and traction control calibration. Wider tyres can improve grip and handling but may increase road noise, fuel consumption and the risk of aquaplaning. Narrower tyres perform better in snow and are generally more fuel-efficient.
Legal Requirements for Tyres in the UK
UK law requires a minimum tread depth of 1.6 millimetres across the central three-quarters of the tread width around the entire circumference of the tyre. Driving with tyres below this limit is an offence that carries a fine of up to 2,500 pounds and three penalty points per tyre. Most motoring organisations recommend replacing tyres at 3 millimetres, as braking performance deteriorates significantly below this level, particularly in wet conditions.
Tyres must also be free from cuts, bulges and other damage that exposes the cords beneath the surface rubber. Mixing tyre types, such as radial and crossply, on the same axle is illegal. While mixing brands is legal, it is recommended to fit matching tyres on each axle for the best handling balance. Run-flat tyres should only be fitted to vehicles designed for them that have tyre pressure monitoring systems.
When to Replace Your Tyres
Beyond tread depth, tyres degrade with age even if they have not been driven on. Most manufacturers recommend replacing tyres that are more than 10 years old regardless of remaining tread, and many suggest a maximum of 5 to 6 years for vehicles driven regularly. The tyre’s age can be found from the DOT code on the sidewall, where the last four digits indicate the week and year of manufacture.
Check your car’s roadworthiness with our MOT checker, or estimate your driving costs using the fuel cost calculator. For tyre safety regulations and vehicle standards, visit GOV.UK.
This calculator provides estimates for guidance only. Results are based on the figures you enter and typical UK averages. This is not professional, financial, medical or legal advice. Always consult a qualified professional for specific guidance.