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Ask any experienced mechanic what separates a car that sails through its MOT from one that fails on a wet Tuesday, and tyre tread depth is usually near the top of the list. It is one of the few safety checks you can carry out yourself in a few minutes with almost no equipment, and it directly affects how your car grips the road when you need it most. Here is how to measure tread depth properly, rather than glancing at the tyres and hoping for the best.

Why tread depth matters more than most drivers think

UK law requires car tyres to have a tread depth of at least 1.6mm across the central three-quarters of the tyre's width, right around the entire circumference. That is a minimum, not a target. Below that depth a tyre is illegal, and the penalties apply per tyre: up to £2,500 and three penalty points for each one that fails, so four worn tyres can turn into a very expensive afternoon.

It is not just about paperwork, either. Tread is what clears water from between the rubber and the road. As depth falls, the tyre evacuates water more slowly, wet braking distances stretch out and the risk of aquaplaning rises sharply. Many tyre specialists reckon wet performance starts to drop away noticeably below 3mm, which is why careful drivers often replace tyres well before the legal limit arrives.

What you need before you start

• A tread depth gauge — a simple plunger type or a digital one, both are pocket-sized and inexpensive
• A 20p coin for a quick roadside check
• A torch for looking into the grooves on a dull day
• Gloves and something to kneel on, because you will be crouching at every corner

Park on level ground, switch off the engine, apply the handbrake and let the tyres cool if you can. Cold tyres are easier to read accurately, particularly at the edges where heat and cornering have already done their work.

How to measure, step by step

• Press the gauge's probe into a tread groove and push the collar flat against the top of the tread blocks. Read the depth from the scale.
• Take readings at three points across the width of the tyre — the inner edge, the centre and the outer edge.
• Repeat at three or more points around the circumference, roughly 120 degrees apart. Move the car forward slightly if you need to reach a different section.
• Avoid measuring on the raised tread wear indicators moulded into the base of the grooves. These sit at 1.6mm, so a reading taken on them will always look like the legal limit.
• Work through all four tyres, and check the spare if you carry one.

If you do not have a gauge to hand, the 20p trick is genuinely useful. Slot the coin into a groove with the outer rim facing you. If the rim is hidden by the tread and you can still see the coin's design, you have more than 1.6mm. If the rim sits clear and proud of the rubber, it is time to book new tyres. The coin only tells you whether you are legal, though, not how close you are to the limit, so a gauge remains the better long-term tool.

Reading the numbers

Write your readings down. The lowest figure is the one that matters, because a tyre only has to fail in one place to be illegal or unsafe. Look for uneven patterns while you are down there:

• Worn on both outer edges: usually long-term under-inflation.
• Worn down the centre: often over-inflation, though powerful front-wheel-drive cars can do this on the front axle too.
• Worn on one edge only: wheel alignment, worn suspension components or a lot of roundabouts.
• Patchy or scalloped wear: balancing or suspension trouble, worth investigating before you spend money on tyres.

If any reading is at or below 1.6mm, the tyre is illegal and needs replacing straight away. Between 1.6mm and 3mm you are legal but in the zone where wet grip falls away, so plan to change them soon. Above 4mm you have plenty of life left, though it is still worth checking pressures and looking for cracks, bulges or screws embedded in the tread.

Making it a habit

Check tread depth once a month and before any long motorway trip, alongside pressures and a quick look at the sidewalls. Rotating tyres front to back, where the handbook allows it, helps even out wear and gets more miles from a set. When replacement time comes, fit matching tyres on the same axle if you can — mismatched rubber can upset handling, especially in the wet. Five minutes with a gauge is the cheapest insurance your car will ever have.