The legal minimum, what else can make a tyre illegal, and two ways to check your own tread in under a minute.
In short: A car tyre is illegal in the UK if the tread depth falls below 1.6mm across the central three quarters of the tread, around the whole circumference. A tyre is also illegal if it has a cut longer than 25mm or deeper than the cords, a bulge or lump in the sidewall, exposed cords, or if it is unsuitable for the way the vehicle is being used. The penalty is a fine of up to 2,500 pounds and three penalty points, for each illegal tyre.
Most drivers know there is a legal tread limit and have a vague sense that it is somewhere around 1.6mm. Fewer know that tread depth is only one of several ways a tyre can be illegal, or that the penalty applies per tyre rather than per car. Four bad tyres is a potential 10,000 pound fine and twelve points, which is a driving ban.
The tread depth rule in full
The requirement is more specific than a single number. The tread must be at least 1.6mm deep:
- across the central three quarters of the tread width, and
- around the entire circumference of the tyre.
Both parts matter. A tyre can have plenty of tread in the middle and be illegal because one shoulder has worn away, and it can look fine on the part you can see while being bald on the section resting against the road. This is why a proper check means measuring in several places and turning the wheel to look right round.
1.6mm is the legal limit, not the safe one Braking distances and wet grip fall away sharply below about 3mm. Most tyre safety organisations recommend replacing at 3mm rather than waiting for 1.6mm. Treat 1.6mm as the point at which you are breaking the law, and 3mm as the point at which you should be shopping.
Two ways to check your tread in a minute
The 20p test
Take a 20p coin and push it into the main tread grooves at several points across the tyre and at a few places around it. If the outer band of the coin is hidden by the tread, you are above the legal limit. If you can see that outer band, your tread may be close to or below 1.6mm and the tyre needs checking properly.
It is a rough check, not a measurement, but it takes seconds and it is far better than guessing.
Tread wear indicators
Every road tyre has small raised bars moulded into the bottom of the main grooves, set at exactly 1.6mm. Look along the shoulder of the tyre for a small triangle, the letters TWI or the manufacturer’s logo, which marks where they are. When the surface of the tread has worn down level with those bars, the tyre is at the legal limit and needs replacing.
A tread depth gauge is more accurate than either method and costs very little, but the 20p test and the wear bars will tell you what you need to know for a quick check on the driveway.
What else makes a tyre illegal
Tread depth gets all the attention, but a tyre with plenty of tread can still be illegal. Any of the following will fail an MOT and can attract the same penalty:
| Defect | What it means |
|---|---|
| Cuts and tears | Any cut longer than 25mm, or 10 percent of the tyre’s section width if that is greater, or deep enough to reach the cords. |
| Lumps and bulges | A bulge in the sidewall means the internal structure has already failed. This is one of the more dangerous defects and the tyre should come off immediately. |
| Exposed cords or ply | If any part of the internal structure is visible through the rubber, the tyre is illegal. |
| Mixed tyre types on an axle | Radial and cross-ply tyres must not be mixed on the same axle, and there are restrictions on mixing across axles. |
| Wrong specification | A tyre that does not meet the load and speed rating the vehicle requires, or is otherwise unsuitable for how the vehicle is being used. |
| Incorrect inflation | A tyre that is not correctly inflated for its use is an offence in its own right, quite apart from the safety consequences. |
Tyre age: not illegal for cars, but it matters
There is no age limit on tyres fitted to ordinary cars in the UK. There is one for larger vehicles: tyres over ten years old are banned from the front axles of lorries, buses, coaches and minibuses, and from all single wheels on minibuses.
For a car, the sensible guidance is that ten years is the practical ceiling regardless of tread. Rubber hardens and cracks over time, and an old tyre with lots of tread can still have poor wet grip and a weakened structure. You can find your tyre’s age on the sidewall: look for a four digit number at the end of the DOT code, where the first two digits are the week and the last two are the year. So 2321 means the twenty third week of 2021.
Penalties, MOT and insurance
An illegal tyre carries a fine of up to 2,500 pounds and three penalty points, and both apply per tyre. At an MOT, a tyre below the legal tread limit or with any of the defects listed above is recorded as a major defect and the vehicle fails.
There is an insurance angle too. If you are involved in a collision while running illegal tyres, an insurer may reduce a payout or dispute a claim on the basis that the vehicle was not roadworthy. That is a lot of exposure for the sake of a tyre.
What causes tyres to wear unevenly
If one part of a tyre is wearing much faster than the rest, replacing it without fixing the cause just means doing it again in a few thousand miles. The wear pattern usually tells you what is wrong:
- Both shoulders worn, middle fine. Underinflation. The tyre is running on its edges.
- Middle worn, shoulders fine. Overinflation. The tyre is riding on its centre band.
- One edge only. Wheel alignment or tracking is out, or a suspension component is worn.
- Scalloped or patchy wear. Usually worn dampers or an unbalanced wheel.
- Fronts wearing much faster than rears. Normal on front wheel drive cars, but rotating tyres and keeping alignment correct evens it out.
If you are replacing a tyre because of uneven wear, it is worth getting the alignment checked at the same time.
Frequently asked questions
What is the legal tyre tread depth in the UK?
The legal minimum for cars is 1.6mm, measured across the central three quarters of the tread width and around the entire circumference of the tyre. Below that the tyre is illegal and will fail an MOT.
What is the fine for illegal tyres?
Up to 2,500 pounds and three penalty points, and both apply per tyre rather than per vehicle. Four illegal tyres could therefore mean a fine of up to 10,000 pounds and twelve points, which is enough for a driving ban.
How do I check my tyre tread with a 20p coin?
Push a 20p coin into the main tread grooves at several points across and around the tyre. If the outer band of the coin is hidden by the tread, you are above the legal limit. If the outer band is visible, the tread may be at or below 1.6mm and needs checking properly.
What makes a tyre illegal apart from tread depth?
A cut longer than 25mm or deep enough to reach the cords, a lump or bulge in the sidewall, any exposed cords or ply, mixing radial and cross-ply tyres on the same axle, a tyre that does not meet the vehicle’s required load and speed rating, and a tyre that is not correctly inflated for its use.
Is there a legal age limit for car tyres in the UK?
Not for ordinary cars. Tyres over ten years old are banned from the front axles of lorries, buses, coaches and minibuses. For cars, ten years is widely treated as a practical limit because rubber hardens and cracks with age even when tread remains.
At what tread depth should I actually replace my tyres?
Most tyre safety organisations recommend replacing at 3mm rather than waiting for the 1.6mm legal minimum, because wet braking performance falls away sharply below about 3mm.
Where are the tread wear indicators on a tyre?
They are small raised bars set at 1.6mm in the bottom of the main tread grooves. Look along the tyre’s shoulder for a small triangle, the letters TWI or the manufacturer’s logo to find where they sit. When the tread is level with those bars, the tyre is at the legal limit.