Tread depths uk legal requirements:

The legal minimum tread depth of the main grooves of car and similar tyres in the United Kingdom and the European Community is 1.6mm. This applies across the central three quarters of the breadth of tread and round the entire outer circumference of the tyre.

Most car type 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.

Tyre tread pattern is designed to give good grip in wet conditions. Generally speaking available grip reduces as the tyre wears or as the road surface water depth increases – you should reduce speed in such conditions. The 1.6mm minimum limit applies to:
Cars and passenger vehicles with up to 8 seated passengers-not including the driver
Motor vehicles & light trailers (including caravans) up to 3500Kg gross vehicle weight

Part-worn tyres
There are many dangers in buying part-worn or ‘second hand’ tyres. Their history is unknown and they may have been removed from a vehicle involved in an accident or have been badly damaged by ‘kerbing’ or similar problems. Repairs may not have been carried out properly, for example to British Standard BS AU 159f. Regulations require a part-worn tyre to be marked “PART-WORN” adjacent to the E e or BS mark, to indicate that it has been properly examined internally and externally before being offered .for sale. Some faults only show up if the tyre has been inflated. Tread depth must be at least 2mm across the whole breadth of tread.

Tyre pressures
Correct tyre pressures are vital for safe handling and optimum braking, grip and tyre life.
Low tyre pressures or overloading will cause increased fuel consumption, more air pollution, shorter tyre life and greater risk of tyre failure.
High tyre pressures may cause reduced comfort, less grip, greater risk of impact tyre damage and reduced stability in braking and cornering.
Pressures should be checked at least every two weeks and only when the tyres are cold. Even a short trip to the local garage will warm up the tyre and raise the pressure. Accurate and reliable gauges are not expensive and will soon pay back their cost. Recommended pressures may vary according to load or speed. Look in the vehicle handbook, or consult your garage or tyre dealer.

Equivalent Pressures

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.

OFFENCES CAN RELATE TO:
Tread depth: a tyre worn below the legal minimum. Mixing: an incorrect mixture of radial and cross ply tyres. Inflation: a tyre not inflated to make it suitable for the purpose to which the motor vehicle or trailer is being put. Cuts: certain long and deep cuts as specified in regulations. Lumps, bulges or tears: caused by separation or partial failure of the tyre structure. Exposed ply or cord Unsuitability: regarding the use to which the motor vehicle or trailer is being put or to the types of tyres fitted to its other wheels.