Tread Depth

New car tyres usually have about 7 to 9 mm of tread. Many countries set a legal minimum of 1.6 mm. Wear bars in the grooves show when the legal limit is reached. A simple tread depth gauge gives the most accurate reading; measure in several grooves across the tyre width.

The grooves remove water from under the tyre so the rubber stays in contact with the road.

Wet braking distances grow considerably as tread wears, long before the legal limit. Different wear across the tread can point to wrong pressure or alignment problems.