Tyres are the vehicle's only point of contact with the road. The actual area of contact between the car and the road through the tyres is small, roughly equivalent to four size eight men's shoes. Bald tyres - 'slicks' - may be fine for a race car on a dry track, but no good at all for road vehicles on a wet road surface. Tyres treads are designed to pump water from the road surface and provide maximum grip. By the time the tread is worn down to the legal limit they will be unable to perform this task efficiently and MUST be replaced immediatly.
In most circumstances tread depth can be used as a suitable indication of when tyres should be replaced - as tyre treads generally wear out before their age effects their performance. However, the age of a tyre will affect its safety and increase the risk of failure, and you should inspect tyres for the signs of aging regularly.
Motor vehicle manufacturers choose the type, make, size, profile, load carrying capacities and speed ratings to match their vehicles, adjusting the tyre pressures to give the optimum grip, ride and handling characteristics. Only change the type of tyres on your vehicle on expert advice from the vehicle manufacturer, or tyre manufacturer.
Tyre pressures should be maintained at or within a very close tolerance of the recommended pressures.
Tyre tread depth and damage
When tyres become worn or damaged they must be replaced. There must, by law, be at least 1.6mm of tread depth across the centre 3/4 of the width of the tread throughout the entire circumference of the tyre. There must be no damage to the tyre body - sidewalls or tread, no bulges or cuts.
It is illegal to mix tyres of a different construction (cross-ply; bias belted or radial) on the same axle. Cross-ply and bias-belted tyres are seldom used on production cars, and are not widely available in the UK.