Used Car Tyre Tread Depth: The 20p Coin Test

The seller taps the tyre with the side of his shoe and says, “Treads are good, mate, loads of life left.”

You nod: partly persuaded, partly unable to see properly. The bonnet is up, the engine is idling, and the conversation is all about the service history and one previous owner.

Yet the only part of the car that ever meets the road receives perhaps three seconds of your attention. You crouch down, peer at the rubber and act as though you understand what you are seeing. You do not get your hands dirty or inspect it properly.

Then, a few weeks later, you are on the M25 in torrential rain, with the wipers working furiously, and the car starts to feel slightly light over standing water. A small, unpleasant thought lands in your stomach: I should have checked those tyres properly.

A three-second check using a coin can change that discussion before it begins.

Tread depth: the quiet deal-breaker on a used car

When buyers arrive to inspect a used car, they usually focus immediately on the paintwork. They glide a hand over the bonnet, notice a scratch no bigger than a grain of rice and suddenly want hundreds of pounds knocked off the asking price.

Meanwhile, the tyres – four black circles of responsibility – often receive almost no attention. But tyre tread depth determines how the car stops in wet conditions, how it grips during an emergency swerve and how it responds when you drive home exhausted through the rain at night.

Tyres can appear “fine” right up to the point when they are not. That is why the coin test matters before you start discussing money.

On a dull Tuesday last autumn, I saw a young couple looking at a used hatchback in a supermarket car park. They had brought along a friend “who knows cars”, which generally means somebody who once watched a YouTube video about head gaskets.

He opened the doors, looked for rust and leafed through the service book like a customs officer. Then he crouched, took a pound coin from his pocket and pushed it into the tyre tread as though feeding a slot machine.

The seller adjusted his stance. The couple moved closer. You did not need to hear anything to understand what the coin had revealed – or how it was about to affect the price discussion.

Tyre tread depth is more than a dull legal figure on a government website. In the UK, the legal minimum is 1.6 mm across the central three-quarters of the tyre and around its entire circumference. Go below that and you are not only breaking the law – you are taking a chance with your braking distance.

Research shows that worn tread can add metres to wet-weather stopping distances. That can be the difference between “a bit of a scare” and “airbags and paperwork”.

Financially, replacing all four tyres on a family car can cost hundreds of pounds. If the existing tyres are close to the legal limit, that is not a minor point. It is a negotiating tool you either use now… or pay for later.

How the 20p coin test works – and why to do it first

The coin test is precisely what it suggests: a fast way to assess tread depth using money rather than an expensive gauge. In the UK, a 20p coin is the usual choice. Take one and examine its outer edge – the raised, ridged border.

Place the coin in the main grooves of the tyre tread, with that border facing down. If the outer rim remains visible above the rubber, the tread is close to the legal minimum or may already be too low. If the rim disappears fully into the groove, there is more usable tread remaining.

Go around the car and repeat the check at several points on every tyre. Check the front, rear, inner edge and outer edge. It takes under a minute, yet it tells you more than the seller saying, “yeah, they’re fine”.

The key is doing it before you become attached to the car. Do not wait until after the test drive, once you are already imagining road trips and Spotify playlists.

On a small Birmingham estate, a reader told me that he used the coin test on a five-year-old SUV he had found online. The interior was immaculate, it drove straight and it came with a fully stamped service book. On paper, it was the ideal school-run tank.

Then he took out a 20p coin. Three tyres easily covered the rim. The fourth? The border remained proud and obvious. He showed it to the seller calmly and without fuss. The conversation immediately moved from “non-negotiable price” to “ok, let’s talk about that”.

There is a psychological element too. Carrying out a coin test in front of a seller silently communicates: I actually know what I’m looking at.

You are no longer simply another buyer dazzled by glossy dashboards and Apple CarPlay. You are inspecting something safety-critical and sufficiently boring that most buyers overlook it. That shifts the balance of power.

Legally, tyres below the minimum tread can result in fines and penalty points for each tyre. In human terms, worn tyres can mean that, in the split second when you desperately need grip, the car simply shrugs its shoulders.

So the little 20p coin does not merely assess tread. It indicates how seriously you take your finances and safety.

Turning a coin into your strongest negotiation tool

This is the straightforward routine used by experienced buyers. Arrive, say hello and walk once around the car. Then – before lifting the bonnet or looking through the paperwork – go directly to a front wheel with a coin already in your hand.

Put it into the groove, inspect it and say nothing for a moment. Check another point on the same tyre, then move to the next one. Continue around the car in a measured, methodical way.

Only after that should you begin the discussion. If the tread is low, there is no need for accusations or drama. Say something along the lines of: “These tyres are close to needing replacement. That’s around £X, so I’d need that reflected in the price.”

At that point, you are not haggling. You are doing maths.

Let us be honest: hardly anyone actually does this every day. Most of us still arrive at used-car viewings a little flustered and already partly overwhelmed.

The coin test gives you one clear task and one clear figure to keep in mind. It also prevents you from falling into the “it looks OK” trap, because worn tyres can still look substantial from the side.

Common errors include checking just one tyre, inspecting only the centre of the tread and overlooking the shoulders where wear may be worse, or believing the seller who says “I just had these done” without evidence.

You do not have to be confrontational. You simply need to be calmly thorough, and slightly determined with that small piece of metal in your hand.

Dealers and private sellers understand that an awkward tyre discussion can stop a sale. That is why a composed, factual approach works well.

“When you show up with a coin and a question, not a complaint, people usually come to the table,” a used-car buyer in Manchester told me. “The tyre doesn’t lie. You’re just reading what’s already written there.”

There is also an easy way to present the issue without making anyone lose face: you are not making an accusation, you are working out a budget. You can even check approximate tyre prices online before you visit.

• Phrase to use: “I like the car, but the tyres are near the limit. I’ll need around £X soon for replacements. Could we meet in the middle?”
• Move to avoid: Allowing the seller to distract you with a fresh MOT while the tread is only just legal.
• Number to remember: 1.6 mm is the legal minimum; many safety experts suggest replacing tyres at around 3 mm.
• Quiet red flag: One brand-new tyre and three almost bald ones – this may suggest the car has not been maintained evenly.

Used properly, that small list in your mind – and the coin in your hand – turns an uncomfortable haggle into a practical, adult conversation.

More than rubber: what the coin test really gives you

Standing in drizzle beside somebody else’s car, crouched down and probing tread grooves with loose change, can feel slightly ridiculous. But that brief, mildly awkward action achieves something subtle: it makes you slow down.

Buying a used car is emotional. Perhaps you are fed up with your old car, worried about money or excited about a fresh start. The coin test acts like a handbrake on that momentum. It makes you examine something unglamorous but non-negotiable.

Once you have done it, your perspective changes. You are no longer merely “hoping it’s a good car”. You have already inspected the part that determines whether that first emergency stop becomes only a story… or a statistic.