Checking tyre tread depth

Tyres are the only part of the car touching the road, and the difference between 6mm and 2mm of tread is car-lengths of stopping distance in the wet.

The law, and the sensible line

The UK legal minimum is 1.6mm across the central three-quarters of the tread, all the way round. Below that: up to £2,500 and three points per tyre, and an MOT failure. But wet-grip degrades sharply below 3mm — most safety organisations recommend replacing there, not at the legal floor.

The 20p test

Push a 20p coin into the main grooves at several points. The coin's outer band is roughly 1.6mm: if you can see that band above the tread, the tyre is at or near the limit. It is a screening test — a proper depth gauge (a few pounds) gives real numbers.

What wear patterns tell you

• Both edges worn — chronic under-inflation.
• Centre worn — over-inflation.
• One edge worn — wheel alignment (tracking) is out, or suspension wear.
• Cracked sidewalls — age. Tyres over ~7–10 years old can fail regardless of tread.

On a used-car viewing, uneven wear is a negotiation point twice over: the tyres need replacing and the cause needs fixing.