What is a tyre’s tread depth?

Tyres are of crucial importance, and so is knowing what tyre tread depth is; we explain all

Tyres are the only thing keeping you in contact with the road, with each tyre having a ‘contact’ patch that’s about the same size as a human hand; that’s not a huge surface area, especially when you consider modern cars can weigh upwards of two tonnes.

Tyres have grooves in them – technically known as sipes – that exist to disperse the water on the road that cars encounter.

Tread depth is the distance from the highest part of the tyre, to the lowest part of the sipe.

What’s the legal minimum tyre tread depth?

Legally, the minimum tread depth every tyre must have is 1.6mm, although it is sensible to replace them before they get this low, with best practice advising replacing tyres once they only have 3mm of tread left.

You can receive three penalty points and a £100 fine if you are caught driving on a tyre that doesn’t have 1.6mm or more of tread, or if the tyre is defective – so keep an eye out for damage such as bulges in the side of the tyre, cracking caused by ageing tyres, or tears in the rubber.

How long will that tyre tread depth last?

Twice as long as half their lifespan.

A huge number of factors affect how long tyres last. Heavier cars tend to wear through tyres more quickly, while a powerful rear-wheel-drive car is more likely to chew through rubber at the rear than a front-wheel-drive hatchback.

As a rule of thumb, though, you should get around 20,000 miles from a set of front tyres in a front-drive car, while the rears might last for 30 or 40,000 miles.d.

How to check a tyre’s tread depth

In a pinch, the border of a 20 pence piece is about 3mm deep, so pop one of these between the treads and see if they reach up to its border.

Given, though, that this is a relatively unscientific method, and that cash is increasingly rare in this modern age, a proper tread-depth gauge, which can cost as little as £2.50, is a sensible investment.

Some tyres also have wear indicators – lines set at the minimum tread depth – that run laterally across the sipes and between tread lines. Once the tread is at the same level as these, it’s time for new rubber.

When should I replace my tyres?

If you’re being fastidious, as soon as they get to 3mm, and certainly once the tread gets to 2mm deep – you don’t want that final 0.4mm worth of rubber to wear down, leaving you liable to penalty points, or the car having poor grip, bad water dispersal abilities, and a higher likelihood of a loss of grip, with the crash that would be likely to bring.

Do note that you should ideally replace tyres once they get to five years old simply because the sun, plus changing temperatures, affect the health of rubber.

What happens if I let my tyres go below the 1.6mm legal minimum tread depth?

As mentioned above, the penalty for driving on a defective tyre is three penalty points, but this can be per tyre, so if all four are bad, you could lose your driving licence.

You could also be fined £2,500 per defective tyre, for a total bill of £10,000.

There is also the risk that tyres with little tread could be a reason for you crashing your car, which could bring even more serious charges and penalties, and potentially even worse consequences.

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