TWI (tread wear indicator)

How to read it

FMVSS 139 defines treadwear indicators as projections within the principal grooves that give a visual indication of how far the tread has worn, and requires at least six of them, spaced about equally around the tire, showing when the tread is down to one sixteenth of an inch (three on tires for rims of 12 inches or less). USTMA and Bridgestone give the same point as 2/32 inch, and Continental as 2/32 inch (1.6 mm). On the sidewall, TyreSafe says the indicators are found by the letters TWI or small triangles; Pirelli says the marking varies by manufacturer.

Also searched as: tread wear bar, triangle on tire, TWI, wear bar.

What the bars mean, in the makers’ and NHTSA’s words

USTMA’s guide says that when a wear bar is flush with the tread surface, the tire is worn out. Bridgestone’s glossary says the indicator defines the legal wear-out point. NHTSA’s tire page puts the replacement point at 2/32 of an inch.

Differences by country or standard

In Great Britain, the MOT inspection manual requires 1.6 mm in the primary grooves across the central three-quarters of the tread and round the whole circumference for cars, and treats the grooves that contain the TWIs as the primary grooves. Some winter tires carry a second, higher set of indicators; Continental says these show when the tire is no longer suited to winter conditions, and gives Austria’s 4 mm winter minimum as an example. British Columbia asks for 3.5 mm on a legal winter tire.

TWI or TREADWEAR?

The TREADWEAR number on a US tire’s sidewall is a different marking: a comparative grade from a government test course, not a depth.