In Europe, the legal requirement is that the tread grooves retain a certain minimum depth, when measured at any Tread Wear Indicator (TWI), located around the circumference of the tyre. For summer tyres, the legal figure is 1.6 mm, while for winter models the advised minimum depth is 4 mm.
It may nevertheless be advisable to change a tyre at an earlier stage if there is a discernible loss of grip or deterioration in handling performance – or if you are driving regularly at high speeds, under extreme weather conditions or over hazardous terrain.
Tyre carcasses should be checked regularly, and replaced if you discover cuts or deformations indicating a weakness in the internal structure. If in doubt, consult your garage or tyre dealer.
Even if not physically worn or damaged, the age of a tyre can also affect its performance. Oils incorporated into the rubber dry out over a period of time, making it less flexible – with the result that grip is reduced and braking distances increased. For more info click here