Current tread depth legislation requires that car tyres must have a minimum of 1.6mm of tread in a continuous band throughout the central ¾ (or 75%) of the tread width and over the whole circumference of the tyre as the image below shows.

However, despite the law, it is generally recognised in the tyre industry that the legal limit is an extreme. Many tyre manufacturers state that they design tyres to function as well at 1.6mm as they do at 9mm (the accepted normal tread depth when new).

Therefore, if a tyre performs as well at 1.6mm as it does at 9mm, what happens at when the tread depth falls below 1.6mm? Does tyre performance drop suddenly? Actually it does not, industry testing has shown that the level of performance in the wet, and it's dry handling characteristics starts to deteriorate when a tyre reaches around 3.5mm in tread depth, well before the 1.6mm legal limit.

Why then the current legal limit of 1.6mm? There are several arguments against a change to a 3mm legal minimum point of change (with 3mm being accepted Europe-wide as the recommended). One is that the sudden change from 1.6mm to 3mm would mean that hundreds of thousands of motorists who are already struggling to keep their cars on the road, would immediately need their tyres replaced. Another is that it would require changing all the tyre moulds in use to increase the tyre wear indicator depth to 3mm. And of course Europe plays a part, as there would not be universal implementation of 3mm tread depth, requiring double standards in production and possibly in policing. Not to mention the environmetal impact of having to handle a sudden increase in waste tyres.

The reality is that since tyres are now a global commodity it would almost require a global adoption of 3mm as a minimum. It doesn't take a rocket scientist to counter the arguments against 3mm, but until the legislation is in place you can make up your own mind, scrape by on 1.6mm, or be safe on 3mm. Your choice.

The braking and grip performance of tyres in wet weather deteriorates considerably once the tread depth reduces below 3mm. This is because the main function of the tread pattern of a tyre is to evacuate water. As the tread depth decreases it gradually loses the ability to evacuate all water from the road surface under the tyre and the car will eventually aquaplane.

Many tyre tests have shown that the wet braking distances of a new tyre compared with a tyre with only 1.6mm of tread left on it are huge and can be the difference between life and death.