The current legal limit is 1.6mm* tread over the central 3/4 of the tyre in a continuous band around the entire circumference of the tyre. The Royal Society for the Prevention of Accidents (RoSPA) recommends that tyres are changed once the tread reaches 3mm in depth.
The depth of tread can significantly affect performance, particularly in wet conditions as shown below:
It is worth noting that legally, it is the driver's responsibility to ensure that the tyre tread depth is not worn beyond the legal minimum limit of 1.6 millimetres.
* Please note that the legal limit given is for the UK and may be different in other countries.