Worn tyres are one of the most common causes of road accidents in the UK - and one of the easiest things to check yourself. The legal minimum tread depth is 1.6mm across the central three-quarters of the tyre, and the penalty for driving on illegal tyres is up to £2,500 and three penalty points per tyre.
The Legal Minimum and the Safety Minimum
The UK legal minimum tread depth is 1.6mm across the central three-quarters of the tyre and around the entire circumference. However, most tyre safety experts recommend replacing tyres at 3mm - at which point stopping distances in the wet begin to increase significantly.
At 1.6mm in wet conditions, a tyre takes roughly 44% longer to stop from 50mph than a new tyre with 8mm of tread. That's around 37 extra metres - more than five car lengths.
Method 1 - The 20p Coin Test
The quickest way to check your tread depth is with a 20p coin. Insert the coin into the main tread grooves of your tyre. If the outer border of the coin (the raised rim) is hidden by the tread, your tyres are above the legal limit. If you can see the outer border clearly, your tyres may be illegal and should be checked professionally.
Check in several places across the width of the tyre and at multiple points around the circumference - tyres can wear unevenly.
Method 2 - Tread Wear Indicators (TWIs)
Most tyres have built-in tread wear indicators - small raised bars moulded into the base of the main tread grooves at 1.6mm depth. When the tread surface is level with these bars, the tyre is at the legal limit and must be replaced.
Look for the TWI marker on the tyre sidewall - it's usually marked with the letters 'TWI' or a small triangle symbol. The indicator bar sits at a point in the groove directly below the marker.
Method 3 - A Tread Depth Gauge
A digital or analogue tread depth gauge costs £3-£10 and gives you an accurate reading in millimetres. Insert the probe into the tread groove, press the gauge flat against the tyre surface, and read the measurement.
Measure in at least three positions across the tyre width (inner, centre, outer) and several points around the circumference for an accurate picture.
Signs of Uneven Tyre Wear
- Centre wear - the middle of the tread is more worn than the edges. Usually caused by over-inflation.
- Edge wear - both edges are more worn than the centre. Usually caused by under-inflation.
- One-sided wear - one edge is more worn than the other. Typically indicates a wheel alignment issue.
- Patchy or scalloped wear - uneven patches around the circumference. Can indicate a suspension problem.
How Often Should You Check?
Check your tyre tread depth at least once a month and before any long journey. It takes under two minutes and could save your life and your licence.
If you notice rapid or uneven wear, have your wheel alignment and tyre pressures checked - driving on correctly inflated, aligned tyres can significantly extend tyre life.