Tyre Tread Depth

Regularly check your tyres to make sure there is plenty of tread. The minimum legal depth is 1.5mm but the more tread you have the better the grip and the safer you'll be.

The image below shows how much a tyre touches a wet road at different speeds. The faster you go, the less effective your tyres will be if the tread depth isn’t up to scratch.

How to measure tread depth

Because it's the only part of the vehicle that grips the road, the depth of tread on your tyres is very important for the safety of your vehicle. Checking your tyre tread depth is easy and only takes a few minutes. You can use a tyre tread depth checker/gauge yourself or go to a local garage and get them to check it for you.

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Requirements for tyre tread depth

Most tyres have moulded tread-depth indicators which are flush with the tyre tread when it has reached the minimum depth. The minimum legal tread depth is 1.5mm within all principal grooves that contain these moulded tread depth indicators. These must be at least 1.5mm around the whole circumference of the tyre.

What are principal grooves and tread depth indicators, and how do I find these?

Principal grooves are the wide grooves in the tyre tread which have the tread-depth indicators located inside them. Any other grooves are secondary grooves which may wear out during the service life of the tyre.

The following shows different tread patterns with tread-depth indicators (and therefore principal grooves) and the width where you should measure marked.

If you find it difficult to find tread-depth indicators, just look along the side wall for a small traiangle or TWI' (tread wear indicator) mark (see below pictures).

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If you can see any worn, damaged or exposed cords (steel from within the tyre) take it to an expert immediately.

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Reference: https://www.nzta.govt.nz/vehicles/choosing-the-right-vehicle/check-your-car-safety-basics/tyres/