The 20p Tyre Test — How to Do It Properly
A practical guide to 20p test tyres for UK drivers, from the CarPal mobile tyre team.
How the 20p test works
A 20p coin has a raised outer band around its edge, roughly 1.6mm wide — conveniently the same as the UK legal minimum tread depth.
Insert the coin into one of the main tread grooves. If the tread covers that outer band completely, you have more than 1.6mm. If any part of the band is visible, the tread is at or below the limit and the tyre needs checking properly.
Doing it properly
Use the main circumferential grooves, not the small sipes or the shoulder channels. Those are shallower by design and will give you a false fail.
Check at least three points around the tyre and across its width — inner, centre and outer. Tyres rarely wear evenly, and the whole point is to find the worst spot rather than a representative one.
Do all four tyres. It takes about two minutes.
What the test cannot tell you
It gives you a pass or fail at 1.6mm and nothing else. It will not tell you that a tyre is at 2mm and will be illegal in a few thousand miles.
It also will not flag sidewall damage, bulges, cracking or age. Those need a visual inspection, and any of them can mean replacement regardless of tread.
When to use a gauge instead
If you want to plan replacement rather than react to it, use a gauge and record the numbers. Knowing you are at 3mm now and were at 4mm six months ago tells you roughly when you will need tyres.
The coin test is a quick safety check. The gauge is maintenance.
Frequently asked questions
Does the 20p test actually work?
Yes, as a pass/fail check at the 1.6mm legal limit. The coin’s outer band is about 1.6mm wide.
Which groove should I test?
The main circumferential grooves. Shallower sipes and shoulder channels give false results.
How many places should I check?
At least three points around the tyre and across the inner, centre and outer grooves.
Can I use a different coin?
The 20p is the one with a 1.6mm band. Other coins do not correspond to the legal limit.
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