Most car tyres have tread wear indicators, usually at least six small ribs across the bottom of the main tread grooves, and when the tread surface becomes level with these ribs the tyre is at the legal limit and must be replaced.
Penalties
The penalties for offences related to the use of faulty tyres on vehicles are very severe.
In the case of any vehicle, except goods vehicles and vehicles adapted to carry more than eight passengers, for every offence there is a fine at level 4 of the standard scale with discretionary disqualification and compulsory driving licence endorsement with 3 penalty points.
Level 4 is currently £2500 and each faulty tyre is considered as a separate offence. Two faulty tyres equals £5000.