Tyres: Reading the Numbers and Knowing When to Replace

The sidewall markings

The size appears as width in millimetres, aspect ratio as a percentage of width, construction letter, and rim diameter in inches.

A load index and speed rating follow, and both must meet or exceed the vehicle manufacturer's specification. Fitting a lower rating can affect insurance and safety.

A four-digit code gives the week and year of manufacture. This is the age of the tyre, which is not the same as how long it has been fitted.

Tread depth

Legal minimums vary by jurisdiction and are typically well below the depth at which wet performance degrades noticeably.

Wet braking distances lengthen substantially before the legal minimum is reached, so replacing earlier is a safety decision rather than an indulgence.

Tread wear indicators sit in the grooves and become flush with the tread at the legal limit.

Check across the width and around the circumference, since wear is rarely even.

Age

Rubber hardens and cracks with time regardless of tread remaining. Many manufacturers recommend replacement after a number of years.

This matters for low-mileage vehicles, caravans, and spare tyres that have never been used.

Inspect for fine cracking in the sidewall and between tread blocks.

Uneven wear as a symptom

Wear on both edges suggests under-inflation; wear in the centre suggests over-inflation.

Wear on one edge only suggests alignment problems, which will destroy the new tyres too if not corrected.

Patchy or scalloped wear suggests worn suspension components.

Fixing the cause before fitting new tyres is what makes them last.

Buying

Fit tyres in pairs at minimum, on the same axle, with matching specification.

Where only two are replaced, fitting them to the rear is generally recommended for stability, regardless of which wheels drive.

Budget tyres differ measurably from premium ones in wet braking and wear rate. Independent test data is available and worth reading.