Checking your tyres takes about two minutes and needs nothing more than a 20p coin. It is the single most useful thing you can do to your car without tools.
The legal limit and the sensible limit
In the UK the minimum legal tread depth for a car tyre is 1.6mm across the central three-quarters of the tyre, around its whole circumference. Below that it is an MOT failure and a fine of up to £2,500 and three penalty points per tyre.
Legal is not the same as safe. Wet-weather stopping distances lengthen noticeably below about 3mm, so most drivers should be planning replacement around 3mm rather than waiting for 1.6mm.
The 20p test
- Push a 20p coin into the main tread grooves at the outer edge of the tyre.
- If you cannot see the outer band of the coin, the tread is above roughly 3mm.
- If the band is visible, get the tyre measured properly.
- Repeat at three points around the tyre and on the inner and outer edges — tyres rarely wear evenly.
What the wear pattern tells you
Worn on both outer edges
Under-inflation. Check pressures against the plate in the driver's door shut or the filler flap.
Worn down the centre only
Over-inflation.
Worn on one edge only
Alignment or a worn suspension component. Fitting new tyres without correcting it just wears the new ones out the same way, usually within a few thousand miles.
Feathered or scalloped patches
Worn dampers or bushes. Again, a tyre problem with a suspension cause.
The other things to look for
- Cracking in the sidewall rubber — age matters as well as mileage, and tyres over about ten years old should be replaced whatever the tread.
- Bulges or blisters in the sidewall, usually from a kerb strike. Replace immediately; the casing is damaged.
- Nails and screws still embedded. Do not pull them out — many are repairable if the puncture is in the central tread area.
- The date code on the sidewall: four digits, week then year. "2320" is the 23rd week of 2020.
Repair or replace
A puncture in the central three-quarters of the tread, under 6mm across, with no sidewall damage, can usually be repaired to British Standard BS AU 159. Sidewall damage, a run-flat that has been driven flat, or a tyre already at 2mm is a replacement.