Tyre Tread Depth: The Legal Minimum and How to Check

The UK legal minimum is 1.6mm — but the wording matters more than the number. Tread must be at least 1.6mm across the central three-quarters of the breadth of the tread, and around the entire circumference.

Two things follow from that sentence, and both catch people out:

• A tyre can be illegal while looking fine. If it is worn below the limit on one part of the circumference — a flat spot, or wear from a sticking brake — the rest having plenty of tread does not help
• The outer quarter on each side is excluded from the measurement, but not from the law. A tyre worn to the canvas on its shoulder is still an offence and still an MOT failure, under the rules covering exposed cord and structural damage

The penalty is up to £2,500 and three penalty points per tyre. Per tyre — four illegal tyres is a potential £10,000 and twelve points, which is a driving ban on its own.

Three ways to check

1. The tread wear indicators. Every tyre has them built in: small raised bars sitting in the bottom of the main grooves, moulded to exactly 1.6mm. Find them using the small triangles, “TWI” markings or the manufacturer’s logo around the edge of the sidewall, then look into the groove at that point.

If the tread surface is level with the bar, the tyre is at the legal limit — not approaching it. It needs replacing now.

2. The 20p test. Insert a 20p coin into the main tread grooves. If the outer band of the coin is hidden, you have more than about 1.6mm. If you can see the band, the tyre may be below the limit and needs measuring properly. Do it at several points around the tyre and across its width, not just once.

It is a rough screen, not a measurement — useful because everyone has a 20p, not because it is precise.

3. A depth gauge. A few pounds, and it is the only one that gives you a number. Measure in several grooves, at three points around the circumference, and on both the inner and outer shoulders. That last part is what finds the wear the other two methods miss.

Why 3mm is the number that actually matters

1.6mm is the point at which a tyre becomes illegal. It is not the point at which it stops working well, and the gap between those two is where most of the risk sits.

Tread has one job in the wet: getting water out of the way so the rubber can touch the road. A tyre displaces an enormous volume of water every second at speed, and it does that through the void space in the grooves. As tread wears, that void volume shrinks — not gradually in proportion, but disproportionately, because the grooves get shallower as well as no narrower.

Independent wet braking tests have repeatedly found that stopping distances deteriorate sharply once tread falls below about 3mm, and that the difference between a 3mm tyre and a 1.6mm tyre in standing water is far larger than the 1.4mm gap suggests. In the dry the difference is minimal, which is exactly why worn tyres feel perfectly fine right up until the day they do not.

The practical advice: budget to replace at 3mm rather than run to 1.6mm. You are giving up a few thousand miles of tyre life for a meaningful improvement in wet braking, and in a British winter that is not a close call.

Aquaplaning

The failure mode worth understanding, because it is the one people do not recognise while it is happening.

When a tyre cannot clear water fast enough, a wedge of water builds ahead of the contact patch and lifts the tyre off the surface entirely. At that point you have no steering, no braking and no acceleration on that wheel — the tyre is riding on water.

What it feels like is not dramatic. The steering goes suddenly light, the engine note may rise, and the car feels as though it is floating. The correct response is counter-intuitive: do not brake and do not steer sharply. Ease off the throttle, hold the wheel straight, and let the speed drop until the tyres bite again.

Three things raise the risk: worn tread, underinflation, and speed. Only one of them is free to fix.

Tyre age, and a rule that does not apply to cars

Rubber degrades whether the tyre is used or not. It hardens, loses flexibility and can develop fine sidewall cracking — so a tyre with plenty of tread can still be past its useful life.

Finding the age: look for the DOT code on the sidewall. The last four digits are the week and year of manufacture — so 3223 means the thirty-second week of 2023.

The common misconception: since 2021 there has been a UK law banning tyres aged ten years or over. It applies to the front axles of lorries, buses and coaches, and to all tyres on minibuses. It does not apply to cars. There is no legal age limit on a car tyre.

That does not make age irrelevant. Many manufacturers suggest considering replacement at five to six years and treating ten as an outside limit regardless of tread — which matters most on a low-mileage car, a caravan, or a spare that has lived under the boot floor since the car was new.

The spare, and what replaced it

Worth two minutes, because most people discover the answer at the roadside.

If you have a full-size or space-saver spare, it needs checking too — pressure, tread and age. A spare that has been under the floor for eight years at half its correct pressure is not a spare. Space savers also carry a speed limit, usually 50mph, marked on the wheel.

If you have a sealant and compressor kit instead, which most new cars now come with, know its limits before you need it. Sealant handles a small puncture in the tread. It does nothing for a sidewall tear, a blowout or a tyre that has come off the rim. The sealant also has an expiry date printed on the bottle, and it is usually four to five years.

If the wear is uneven, tread depth is not the problem

A tyre wearing evenly across its width is doing its job and will simply need replacing eventually. A tyre wearing unevenly is telling you about something else, and replacing it without fixing that just starts the same clock again.

• One edge only, smooth — camber, or a bent component
• Sharp one way, smooth the other under your palm — toe. The fastest-acting wear pattern there is
• Both edges, centre fine — persistently underinflated
• Centre only — persistently overinflated
• Scalloped high and low patches — the dampers, not the tyre

Reading the wear pattern to name the angle is worth doing before buying tyres rather than after — a misaligned car can scrub a new front tyre out in a few thousand miles, and an alignment costs a fraction of a pair.

What does it cost?

Item	Typical Birmingham range
Tread depth check	Usually free
Depth gauge (buy once)	£3–£10
Front tracking (toe only)	£35–£50
Four-wheel alignment with printout	£55–£90
Puncture repair, where the damage allows	£25–£40

Not every puncture can be repaired. Repairs are permitted only within the central portion of the tread and within a size limit — a sidewall puncture, or damage near the shoulder, means a new tyre regardless of how small the hole is.

Why Birmingham tyres wear unevenly

Local wear patterns are consistent enough that the cause is usually guessable from the tread.

Inner-edge wear on a car that lives on terraced streets is almost always accumulated kerb contact — no single memorable impact, just months of parking nose-in at walking pace. It is the most common pattern here, and it is also the most dangerous, because the inner edge is the part you cannot see without turning the wheel to full lock and looking.

That is the practical takeaway. A glance at the outside of a tyre tells you very little. Turn the steering to full lock, look at the inner shoulder of each front tyre, and run a hand across it. A great many cars in this city are legal on the visible three-quarters and illegal on the quarter nobody checks.

Sharp-edged potholes add a second local risk that has nothing to do with tread: a sidewall bulge. A bulge means the internal cords have been damaged, the tyre can fail without warning, and it cannot be repaired. What to check after a pothole strike covers it, and the sidewall is the first thing on that list.

When should you call a garage?

Today if you can see a tread wear indicator level with the tread surface, if there is a bulge, cut or exposed cord in a sidewall, or if a 20p shows its outer band anywhere on the tyre.

Before you buy tyres rather than after, if the wear is uneven. Fitting new rubber to a car with a geometry fault is the single most common way people spend twice.

Ask for the inner edges specifically at your next service. They are checked properly with the wheels off, and they are where the wear hides.

Worth knowing: tyres are consistently one of the largest single causes of MOT failure, and edge wear is a substantial share of that. An advisory this year is frequently a failure the next.

Ring and tell us what you are seeing and roughly how old the tyres are, and we will tell you whether it is a repair, a pair, or a geometry problem first. Tyres, wheel alignment and balancing in Birmingham, seven days a week, and we check the suspension for play before setting any geometry.