Tread depth is the obvious one, but it is not the only thing that decides when a tyre needs replacing. Age, damage and uneven wear all end a tyre's life, sometimes while the tread still looks perfectly healthy.
Tread depth
The UK legal minimum is 1.6mm across the central three quarters of the tread, right around the circumference. Below that you risk three penalty points and a fine per tyre, so a car with four bald tyres is a licence gone.
Legal and safe are not the same thing though. Wet braking distances lengthen sharply below about 3mm as the grooves lose the volume to clear water. Most safety bodies, and we, suggest planning replacement at 3mm rather than waiting for 1.6mm.
Quick check: push a 20p coin into a main groove. If you can see the outer band of the coin, you are near or below 3mm and should start planning.
Age
Rubber degrades whether the car moves or not. Heat, sunlight, ozone and time all harden the compound and cause fine cracking, which reduces grip long before the tread runs out.
- 6 yearsStart inspecting closely each year, even if tread looks fine
- 10 yearsReplace regardless of tread depth or appearance
Find the age in the four digit DOT code on the sidewall. The first two digits are the week, the last two the year, so 2723 means week 27 of 2023. Low mileage cars, caravans, classics and spare wheels are where age catches people out.
Damage
Some damage means replacement, not repair:
- Sidewall bulges. The internal structure has failed. The tyre can let go without warning. Replace immediately.
- Cuts or cracks deep enough to expose cords.
- Sidewall damage of any kind. Sidewalls cannot be safely repaired.
- Objects embedded in the tread. Sometimes repairable if in the central area and under 6mm, but get it assessed rather than pulling it out yourself.
Uneven wear
How a tyre wears tells you what is wrong with the car. Fitting new tyres without fixing the cause just wears out the new ones too.
- Both edgesUnder inflation. Check pressures monthly.
- Centre stripOver inflation.
- One edge onlyAlignment or suspension geometry out.
- Patchy or scallopedWorn shock absorbers or unbalanced wheels.
Do I have to replace all four?
No, but replace in pairs across an axle at minimum. Two tyres with different grip levels on the same axle unbalance the car under braking.
If you are only replacing two, put the new pair on the rear, whichever wheels drive the car. Worn rears let go first in the wet and cause oversteer, which is far harder to catch than the understeer you get from worn fronts.
What about the spare?
Worth a look if you have one. Spares sit for years untouched, lose pressure, and frequently fail the 10 year age test long before anyone checks. A spare you cannot use is just weight in the boot.