What it does
The grooves in a tyre push water out from under it. As the tread wears down, the grooves get shallower, and on a wet road the tyre grips less and stops later. This tool takes your measured depth, the country and the type of vehicle and tells you plainly: Legal, Close to limit, or Replace.
It also shows a small gauge, the legal limit for your choice and a few simple ways to check without a gauge. Check the age of the rubber with tyre age, the pressure with tyre pressure and what wet roads do to braking in stopping distance.
How to use it
- Pick your country and the kind of vehicle.
- Pick the unit. A tread gauge in mm is cheapest; in the USA the usual unit is 32nds of an inch.
- Measure in the main groove at three places across the tyre and use the lowest number, then type it in.
- Read the verdict, then try the coin and wear-bar checks below if you have no gauge.
What each setting does
- Country: sets the legal minimum and, in the USA, the unit (32nds of an inch).
- Vehicle: car, motorcycle or truck. Some countries allow less on motorcycles and big trucks.
- Unit: mm or 32nds of an inch. The answer is the same either way.
Worked example
A UK car tyre measures 1.2 mm. The limit is 1.6 mm, so the answer is Replace. A US tyre that measures 5/32 inch is 3.97 mm: above the 2/32 inch limit and above 3 mm, so it is Legal, though the wet grip is already starting to fade.
FAQs
What is the best depth to replace at? Most tyre-safety advice says replace by 3 mm (4/32 inch) for wet grip, and keep 4 mm (5/32 inch) or more on winter tyres.
What are the wear bars? Small raised bars inside the main grooves, 1.6 mm high. When the tread is level with them, the tyre is at the common legal limit. Look for the triangle marks on the sidewall that point to them.
Do coin tests work? They are rough. A US penny with the top of Lincoln’s head showing means 2/32 inch or less; a quarter with Washington’s head touching the tread means 4/32. In the UK, if the outer band of a 20p coin is visible in a groove, the tread is likely under 1.6 mm. A gauge is better.
Which tyre do I measure? All of them, and the spare. Uneven wear points to pressure or alignment problems.
Not safety advice. When in doubt, ask a tyre shop.
Where the numbers come from
- Legal minimums as checked November 2026 (check the official source for your area): UK cars 1.6 mm across the central three-quarters of the tread, UK motorcycles over 50 cc and vehicles over 3.5 tonnes 1 mm; most US states 2/32 inch (a few differ; federal rules want 4/32 inch on the steering tyres of large trucks); Canada varies by province, commonly 2/32 inch; India and the Gulf states are commonly cited as 1.6 mm. 1 inch = 25.4 mm, so 1/32 inch = 0.794 mm. The advice to replace by 3 mm (4/32 inch) for wet grip, and keep 4 mm (5/32 inch) or more on winter tyres, is the common tyre-safety guidance. The 1.6 mm tyre wear bars are a worldwide tyre design standard. Coin tests are rough guides only. Drawings are fibub's own.
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Last updated 2026-11-12.