AIUI, when TWI's are no longer below the main tyre tread, it is a visible warning that the tyre has reached the legal depth limit of 1.6mm. Of course it is unlikely that all TWIs will give the same reading. Several regulars on here prefer to change tyres long before that. I presume that MoT testers may use TWIs as a quick check rather than using a depth gauge - anyone confirm ?

The relevant measurement for MOT purposes is from the outer surface of the tyre to the bottom of the tread over 75% of the width of the tyre.

The wear bars are an indication of the 1.6mm legal requirement, but should not, necessarily, be relied upon.

Edited by leaseman on 02/02/2025 at 14:28

When I use my digital depth gage and measure from the tyre surface to the top of the rubber wear indicators I have 2.1mm..

When I measure from the tyre surface to the bottom of the grooves (across 3/4 of the centre) i get 4.0mm.

The TWIs are set at 1.6mm, so you're measuring 1.6 + 2.1 =3.7mm. You're measuring 4.0mm otherwise.

Do you think you can differentiate 0.3mm accurately?