New tyres start life with a full 8mm across 100% of the tread, whereas part worn usually have between 2 to 3mm across most of the tread. The UK limit is 1.6mm so in very little time the part worn will be illegal and resulting in a fine and a mandatory 3 points on your license for each illegal tyre.
TyreSafe an industry body bought 50 part worn tyres and had them independently inspected of which 49 were found to be illegal.
Birmingham Trading Standards carried out a similar inspection and found that 90% of the part worn tyres purchased were also found to be illegal.
Department of transport figures for a single calendar year showed that illegal and defective tyres were responsible for more than 1210 road casualties including 18 deaths bringing the total deaths on the UK roads over the last 5 years to 164 directly related to illegal and defective tyres.