A Driving Instructor's Blog

This is an OLD story from 2011. The campaign referred to is outdated now.


The Westmorland Gazette reports that Cumbria police are launching a month-long campaign to get drivers to check that their tyres are safe and legal before winter weather sets in (note that this article is from Winter 2011).

10 pence coin

Obviously this is a highly laudable campaign. But one thing is niggling me about the story. It talks of the “20p test”, and how you can use a 20p coin to gauge the depth of your tyre tread. This isn’t quite right.

The official minimum specification for tyres on a car is that they should have not less than 1.6mm of tread across the middle three-quarters of the tyre’s width, and this should apply all the way around the tyre. Also, there should be no damage (such as cuts and gashes) or bulging on the sidewalls.

You can measure your tread depth using a suitable depth gauge, and these are available for a few pounds from any Halfords or motorists’ store. They’re very simple – just a plunger that you push into the tyre tread and a scale where you can read off the depth in millimetres. I definitely recommend that you buy one if you’re serious about driving safely.

At a pinch, you can use a 10p (NOT 20p) coin to check against the legal limit. That ring of small dots is about 1.6mm away from the edge of the coin, so if you poke the coin into your  tyre tread the dots should be hidden.20 pence coin If they aren’t – or if they are anywhere near not being hidden – then you must get your tyres fixed to remain legal. It’s worth bearing in mind that garages probably won’t MoT your car if it has less than 3mm of tread on any tyre, and it’s also my understanding that tyre manufacturers make no warranties below about 3mm anyway. (Edit: 2013 10p pieces don’t have dots anymore – I’m not sure if that’s temporary or a permanent part of the design from now on. If you use this method to measure your tyres, keep an old 10p piece somewhere handy.)

To be fair to the article above (and the Cumbria police), they don’t mention the minimum legal specification – theirs is a safety campaign with its own criteria – so they talk of using the 20p coin to measure your tyre tread depth. That rim with the writing on it is about 2.5mm wide, so if you poke the 20p into the grooves and you can see the edge of that border with the writing on it above the tyre then you’re unsafe inasmuch as you have less than 2.5mm of tread, and your road holding will be impaired as a result.

If you just comply with it, you’re less than 1mm away from being illegal. It’s very dangerous ground the Cumbria police department is treading, as it is actually saying that you ARE safe if you have 2.5mm of tread, when in fact anything less than 3mm is the usual limit of acceptability.

It smacks of launching a campaign but being afraid of how it will be received by the many people you’re likely to catch out with it – so lessen the impact by widening the acceptable limit and just give people a stern talking to!

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