Is your tread too thin? The wear bar and the coin test

A tyre that has lost its tread neither grips the road nor clears water. The US National Highway Traffic Safety Administration (NHTSA) treats a tyre as unsafe once the tread is down to 2/32 of an inch, roughly 1.6 mm. Here is how to check yours.

1. Find the wear bars

Every tyre has small raised bars moulded into the grooves between the tread blocks, set at the minimum depth. Look at them in several places around the tyre.

• If the tread stands clearly above the bars, the tyre is still fine.
• If the tread is level with them, it is time to replace it.

2. The coin test

The American method: put a penny into the groove with Lincoln’s head upside down and facing you. If you can see the top of his head, the tread is below the limit. Without a penny, the wear bars do the same job, as does a tread depth gauge that reads in millimetres.

3. Check all four

Tyres rarely wear evenly, and one can reach the limit before the others. Check each tyre separately, in the middle of the tread and at both edges.

4. Make it monthly

NHTSA advises checking the tread at least once a month, when you check the pressure:

• Check pressure with the tyres cold, after the car has stood for at least three hours.
• The right pressure is the one on the vehicle’s placard or in the owner’s manual, not the figure moulded on the tyre.
• The car’s tyre pressure warning only reacts to a large drop; it does not replace checking by hand.

Photo: a close view of worn tyre tread. Photograph by Shixart1985, CC BY 2.0 via Wikimedia Commons.

Source: “Tires”, US National Highway Traffic Safety Administration (nhtsa.gov), read 17 October 2026.