The short answer
The legal minimum is 1.6mm across the central three quarters, but wet braking deteriorates badly below about 3mm. Age matters too: rubber hardens after about six years.
- Check tread across the whole width, not just in the middle. Uneven wear points at pressures or alignment.
- 1.6mm is the legal minimum, and the penalty is significant per tyre plus penalty points.
- Consider replacing at around 3mm, where wet braking distances start to lengthen considerably.
- Check the date code, a four-digit number on the sidewall giving week and year. Tyres over about six years old harden regardless of tread.
- Look for cracking, bulges and cuts in the sidewall, any of which means replace immediately.
What will ruin it
A bulge in a sidewall means the internal structure has failed, usually from a pothole or a kerb. It can blow out without warning at speed. Replace it now rather than at the next service.
Why it works
Tread exists to move water out from under the tyre. As it wears there is less space for the water to go, so the speed at which the tyre aquaplanes falls, which is why wet grip degrades long before the legal limit.
Tipking is general household information, not professional advice. Test anything on a hidden patch first. Full disclaimer.