1. The law states that there should be a minimum tread depth of 1.6mm. It must be measured across the central 3/4 of the tyre and all the way round it too.
What does this mean in practice?
The first part of the law says there must be a minimum of 1.6mm of tread. This means that the depth of the grooves MUST BE NOT LESS THAN 1.6mm. (see image below)
2. The second part of the law says that the 1.6mm measurement must be across the central 3/4 of the tyre. This means that at any point across the main part of the tyre, the tread depth MUST BE NOT LESS THAN 1.6mm.
3. The third part of the law says that the 1.6mm measurement must be all the way round the tyre. This means that there MUST BE NOT LESS THAN 1.6mm of tread all the way round the tyre.
WHY IS THIS?
Its all about safety. Independent tests commissioned by RoSPA have shown that your stopping distance increases greatly when the depth of your tread get down to the legal minimum.
The legal minimum is just that, an absolute minimum, your stopping distance in the wet will start to increase dramatically when your tread depth gets below 3mm, of course this depends on what surface you are driving on and the temperature as well.
RoSPA recommend that for maximum safety that you change your tyres when the depth gets to 3mm.