1.6mm is the legal limit. It is not the safe one

What the law says

1.6mm across the central three quarters of the tread, all the way round. Below that it is an MOT failure, three penalty points and a fine, per tyre.

It is a floor, not a recommendation, and it was set as a minimum rather than as the point at which a tyre stops working well.

What happens in the wet

Tread exists to move water out from under the tyre. Less tread means less water moved, and the effect is not linear — it falls away sharply near the bottom of the range.

Braking distances from motorway speeds on a wet surface increase substantially between 3mm and 1.6mm. Not by a car length. By considerably more than that.

The tyre does not feel different in the dry right up until the day it matters.

Why 3mm is the number people use

Most manufacturers and safety bodies suggest replacing at 3mm rather than running to the limit, and that is the figure we work to on fleet vehicles.

It costs a little more per year and it keeps you well clear of both the legal line and the point where wet performance collapses.

How to check it properly

A 20p coin: if the outer band is visible when you insert it into the main grooves, the tread is near or below the limit. It is crude and it is better than nothing.

Check in three places across the tread and at several points around the tyre. Uneven wear tells you about pressures and alignment as much as about the tyre.

What uneven wear is telling you

Worn in the middle means over-inflation. Worn at both edges means under-inflation. Worn on one edge only usually means alignment.

Replacing the tyre without fixing the cause just means doing it again sooner.

Where it goes from worse to sudden

Reduced grip in the wet is gradual. Aquaplaning is not: below a certain amount of tread at a certain speed on a certain depth of water, the tyre stops touching the road and the steering goes light.

Tread depth is the single biggest thing you control in that equation. Speed is the other, which is why the advice in standing water is to slow down rather than to trust the tyres.

Grip fades gradually. Aquaplaning arrives all at once, which is why people are surprised by it.

The one nobody checks

If the vehicle has a spare, it has tread and a pressure and an age, and almost nobody looks at any of them until the day it is needed.

A spare that has sat at fifteen psi for four years is not a spare. We check them on every visit where one is fitted, because finding out at the roadside is the worst possible time.

Space-saver spares have their own speed limit, usually 50mph, printed on the wheel. It is not advisory.