The law states that:
Car tyres should have a minimum tread depth of 1.6 mm across the central three quarters of the face of the tyre (the part of the tyre which makes contact with the road) and around its full circumference. The edges must also show some tread. The walls of the tyres must not have any cuts or bulges rendering them dangerous.
Tyre manufacturers nowadays incorporate “tyre wear indicators” which become visible when the tyre wears. These are set at 1.6 mm depth between the grooves of the tread. Whilst this is the legal limit it has been found to be a lot safer to replace tyres at 3 mm as tests have proven that it takes longer for a vehicle to stop with the lower level of tread. As a rough guide if one inserts a 20p coin in to the groove of the tread and the outer band of the coin is hidden then the tyre is still legal.
Where possible always opt for the best quality tyres that you can afford. Budget tyres, whilst cheaper and may look the same, are manufactured from a weaker rubber compound and will take longer to stop a vehicle. Also avoid purchasing “part worn” tyres as they may come from a vehicle which was in a collision.
The penalty for driving with an illegal tyre is currently a fine of up to £2,500 and three penalty points per tyre. Having four illegal tyres would also lose you your licence.