The legal minimum tread depth for passing a Warrant of Fitness Test is 1.5 mm over 75% of the tyres width. This is the absolute minimum and it proves to be barely adequate in heavy rain conditions. Ideally, tyres should be replaced when their tread gets down to 3 mm; below which the treads effectiveness in heavy rain is severely reduced.
The big problem in giving WOF’s to vehicles with 1.5 mm over only 75% of the tread area is that the other 25% of tread area maybe be smooth rubber. Ok, that does not seem a big problem, but no one knows for sure how close the steel belts are to the surface. This gives rise to situations where the motorist can leave Auckland and drive to Hamilton on WOF compliant tyres and arrive in Hamilton with steel belts showing over 25% of the tyre; clearly an illegal, insurance voiding situation.
Bettertyres contends that the legal minimum tread should be no less than 1.5 mm over the entire width of any tyre. Also, various overseas agencies recommend that tyres should be replaced when the tread is down to 3 mm as a tyres effectiveness in wet weather rapidly declines below that point.