In the UK car tyres must have at least 1.6mm of tread on them – any less is illegal. But is it safe to let your tyres wear right down to the limit, or should you be changing them earlier?
Lots of motorists have experienced the scenario – you take your car for a service, or an annual MOT test, and are told “your tyres are still legal but it won’t be long before they need replacing…” And you happily drive home, grateful to have put a little more expenditure off for a while.
Is it really worth putting it off, however? A tyre may be legal, but does it still offer all of the safety performance that a brand-new one will? After all a brand-new tyre typically has a tread depth of 8 to 9mm, around five times more than the legal limit, and numerous tests carried out by the automotive industry have shown stretching a tyre to its legal limit is anything but a good idea.
The legal tyre tread depth limit
Firstly, what does the UK legal limit actually mean? Well the tread depth, the grooves in the tyre, must be at least 1.6mm across the central 75% of the tyre’s width, around its complete circumference.
Importantly, that’s only the UK law – the rules vary in different countries. In Germany, for example, the legal limit is 3mm, almost twice the depth of a legal UK tyre and something not to be forgotten if you are planning to take the Eurotunnel and drive across Europe.
Quite simply, the less tread your car’s tyres have on them, the longer it will take to stop, particularly in wet conditions – in other words, much of the time here in the UK.
The grooves are principally there to clear water from the road surface under the tyre and to help to maintain a good grip on the road. So the deeper the tread, the more water it can clear.
This reduces the risk of aquaplaning, which is when the tyre breaks contact with the road surface and instead travels on the film of water between tyre and road. If this happens, you lose control of the car’s steering, acceleration and braking – and no amount of pounding the brake pedal will slow the car down.
Measuring the impact of worn tyres
To illustrate the difference in braking performande, German tyre manufacturer Continental carried out a series of wet braking tests with new and worn examples of its tyres back in 2020.
Braking from 80km/h (just under 50mph) to a standstill, a brand-new set of tyres with 8mm of tread depth stopped the car in 27.6 metres.
With the tread worn down to the German legal limit of 3mm, it took another 2.2 metres to stop the car. And at the UK legal limit of 1.6mm, the car took 34.4 metres – an extra 6.8 metres compared to new tyres. That’s more than an extra car length.
The Continental tests highlighted another sobering fact – if we imagine that a car with new tyres braking from 50mph was just able to stop in time to avoid hitting an obstacle, then a car with tyres of only 1.6mm tread depth would hit that obstacle at 22mph.
A 22mph impact is fast enough to cause serious damage and injury. That obstacle could be a wall, another vehicle, or a person…
Minimum requirement vs best practice
One of the world’s leading safety bodies is Euro NCAP, which tests most new cars sold in the UK and across Europe for their safety levels – both in avoiding an accident and coping with an impact. The tests are above and beyond the minimal legal requirements set by the EU and UK, so it’s entirely legal for a car with a zero-star safety rating from Euro NCAP to be sold here in the UK. But we wouldn’t recommend buying it.
The 1.6mm rule is basically the tyre equivalent of a zero-star safety rating. It’s legal, but not recommended.
Most industry studies recommend that the longest you should put off changing your tyres is when they reach 3mm of tread depth. You won’t be getting the performance of a new tyre at that depth, but below 3mm a tyre’s stopping performance falls off a cliff.
Checking your tyre tread depth
How do you know how much tread depth your car tyres have? Well checking tread depth is not a difficult process and there is certainly no excuse for driving on illegal tyres.
Modern tyres have indicators built into the grooves of the tread, which show when you reach the 1.6mm limit, though a much more accurate method of checking is to use a tyre depth gauge, available for very little cost at any automotive aftermarket supplier.
Even simpler is to insert a 20p piece into the groove – if you can’t see the wide outer rim of the coin, then you have at least 3mm of tread depth which is well within the legal limit. If you can see the rim you need to make a more accurate check.
In addition to the safety compromises, there are big legal consequences for going below that 1.6mm. If you are stopped by the police and found with illegal tyres, you could be subject to a £2,500 fine and three penalty points on your licence – per tyre. In other words, if all four tyres are under the legal limit, that’s potentially £10,000 and 12 points on your licence – which triggers an automatic driving ban…