Tread depth is the one thing on a car that people treat as a pass or fail. Above 1.6mm it is legal, below it is not, and that framing quietly suggests 1.7mm is fine. It is not fine. It is legal, which is a statement about enforcement rather than about stopping distance.

The number worth working to is 3mm, and it is worth understanding why rather than taking it as a rule.

What The Tread Is Actually For

On a dry road, tread does very little — a slick has more rubber in contact and grips better, which is why racing tyres have no pattern. Tread exists for one job: getting water out from under the tyre.

Driving through standing water, the tyre has to displace it, and the only route for that water is through the grooves. A deep groove has volume to move water into. A shallow one does not, and once the grooves cannot take the water away fast enough, a wedge of water builds under the leading edge of the contact patch and lifts the tyre off the road.

So tread depth is a measure of water capacity, and it falls away as the tyre wears. That is why nothing changes in the dry as a tyre goes down, and why the wet performance drops off so noticeably.

Where 3mm Comes From

Wet braking distance does not decline evenly as tread wears. It holds up reasonably well from new down to around 3mm, and then it deteriorates sharply through the last part of the tyre’s life. The drop between 3mm and 1.6mm is much larger than the drop between 8mm and 3mm, even though the amount of rubber lost is smaller.

Resistance to aquaplaning behaves the same way, only worse — the speed at which a tyre will start to aquaplane through standing water falls off steeply once tread gets shallow. A tyre at 2mm in heavy rain on a rutted A-road is a genuinely different vehicle to drive.

Hence the sensible working limit: change at around 3mm rather than running to the legal figure, and if you are approaching winter with tyres near the limit, change them before rather than after. It costs the same money slightly sooner and buys the grip in the months you need it.

Checking It Properly

The 20p test is fine as a rough screen — insert a 20p coin into a main groove, and if the outer band of the coin is visible, the tread is getting close to the legal limit. What it does not do is tell you when you are at 3mm, so a cheap tread depth gauge is a better buy.

Two things people miss when checking:

• Check across the width, not just in the middle. The legal requirement applies across the central three-quarters of the tread and around the whole circumference. A tyre can be legal in the middle and bald on the inner shoulder, which is both an MOT failure and a sign of something else being wrong.
• Check all four, and the spare. Wear is rarely even between axles, and a spare that has sat in the boot for eight years may be legal on tread and useless on age.

Tread wear indicators — the small raised bars in the base of the grooves — sit at the legal minimum. If the tread is level with them, the tyre is finished and has been for a while.

Age Matters As Much As Tread

Rubber is a perishable material. It hardens with age and with exposure to heat, sunlight and ozone, whether the tyre is being used or not, and a hard tyre grips badly — particularly in the cold, where a modern compound relies on staying flexible to work.

That is why a five- or six-year-old tyre with plenty of tread can be worse on a frosty morning than a newer tyre with less. It is also why caravan and trailer tyres fail: they have full tread and have been sitting in the sun for a decade.

The age is on the sidewall as a four-digit code, usually in an oval. The first two digits are the week, the last two the year — so 3223 means the thirty-second week of 2023. It is worth looking at, especially on a used car, where a bargain can turn out to be wearing tyres older than the last two owners.

There is no MOT age limit for tyres on ordinary cars, though there are rules for some larger and passenger-carrying vehicles, and those rules change — so check the current position rather than assume. As a practical matter, a tyre approaching ten years old should be replaced regardless of tread, and one that is cracking or crazing on the sidewall should be replaced now.

Uneven Wear Is A Message, Not A Tyre Problem

A tyre wearing unevenly is telling you something about the car. Replacing it without fixing the cause means doing it again in a few thousand miles.

• Worn on the inner edge only — usually tracking or camber out of adjustment, often after hitting a kerb or a pothole. Extremely common, and the potholes around here do not help.
• Worn on both outer edges — persistent underinflation. Check pressures monthly and check them cold.
• Worn in a band down the centre — overinflation.
• Feathered, with one side of each tread block sharper than the other — toe misalignment, and you can often feel it with a palm run across the tread.
• Patchy or scalloped wear — worn dampers or a bush letting the wheel move about, so the tyre is not held steadily on the road.

Any of these is worth a look at the suspension and a check of the alignment. A wheel alignment after new tyres is money well spent if the old ones wore unevenly, and a suspension and steering inspection is the way to find the bush or damper causing it.

Part-Worn Tyres

Part-worns look like a saving and rarely are. Consider what you are buying: a tyre with an unknown history, possibly at 4mm, at a price that is a decent fraction of a new budget tyre — so you are paying a large share of the cost for a small share of the life. On cost per millimetre of usable tread, new tyres are almost always cheaper.

Then there is what you cannot see. A tyre may have been run flat, kerbed hard, punctured and repaired badly, or stored outside for years. Internal damage is invisible from the outside and is exactly what causes a sudden failure at speed.

There are legal requirements that part-worns are supposed to meet before sale, including a minimum tread depth above the legal limit and being marked as part-worn, and surveys of the trade have repeatedly found sellers who do not comply. If you are going to buy them, check the markings and the date code and look inside the tyre — but for most people a new budget tyre from a known maker is the better decision.

All-Season Or Winter Tyres Up Here?

Summer tyres lose flexibility below about seven degrees, which in the High Peak covers a great deal of the year on an early morning. Cold-weather compounds stay pliable and have tread patterns that bite into snow and clear slush.

Whether they are worth it is genuinely a question of where you live and what you have to do:

• Full winter tyres make sense if you are up in the hill villages, you have to get out every morning whatever the weather, and you are willing to store and swap a second set. On snow and ice the difference is not marginal, it is the difference between going and not going.
• All-season tyres are the sensible compromise for most people around Buxton, Chapel, Whaley Bridge and the villages above the A6 — noticeably better than summer tyres in the cold and the wet, no swapping, no second set of wheels, a small penalty in the dry.
• Good summer tyres with plenty of tread are adequate if you are lower down and mostly on gritted main roads — Crewe, Stafford, Derby, the Cheshire plain.

Whatever you fit, fit them in fours, or at minimum as a matched pair on the same axle. Mixing tyre types across an axle gives you different grip on each side of the car, which is worse than either tyre would be on its own.

The Short Version

Change at 3mm, not 1.6mm. Check the date code as well as the tread. Look at the inner edge, not just the middle. Treat uneven wear as a suspension or alignment fault rather than a tyre to replace. And if your tyres are marginal going into November, do it now — the months when grip matters most are the months you are about to drive through.