TWI and the tread-wear indicator triangle
TWI marks where the tread-wear indicators are: bars moulded in the main grooves that show when the grooves are only 1.6 mm deep. When the tread is level with a bar, the tyre has worn down to that depth.
What to do with it
Follow a TWI mark or triangle down to the groove and look at the bar. If the tread is flush with the bars, the grooves are down to about 1.6 mm. Continental gives 1.6 mm as the minimum tread depth; the legal minimum where you drive is set by that country’s law.
- What it looks like
- The letters TWI, or a small triangle, on the shoulder where the sidewall meets the tread, repeated around the tyre. Each points to a raised bar across the bottom of a main groove.
- Set by
- UN Regulation No. 30, paragraphs 2.27 and 6.3.1–6.3.3; 49 CFR 571.139, S3 and S5.4
In detail
- UN R30 6.3: at least six rows of indicators around the tyre (four on rims of 12 inches or less), showing “when the tread grooves are no longer more than 1,6 mm deep”, with a tolerance of +0.60/−0.00 mm.
- US FMVSS 139 S5.4: at least six indicators showing wear to one sixteenth of an inch (1.6 mm); three on rims of 12 inches or less.
- The TWI letters or triangle on the shoulder are the makers’ locator; the regulation requires the bars, not the letters. Pirelli notes that the locator varies by manufacturer.
- Continental notes that several winter tyre models are also equipped with winter tyre wear indicators.
What it does not mean
The mark on the shoulder is not the indicator itself and does not measure anything; reaching the bar is not a sign of good tread left.
Markings that go with it
Sources
Each document was fetched and read on the date shown; the clause is the one that sets this marking.
- UNECE, as published in the Official Journal of the EU: UN Regulation No. 30 — Uniform provisions concerning the approval of pneumatic tyres for motor vehicles and their trailers, Revision 3 (OJ L 201, 30.7.2008) — paragraphs 2.27 and 6.3.1–6.3.3: “The pneumatic tyre shall include not less than six transverse rows of wear indicators … The tread-wear indicators must provide a means of indicating … when the tread grooves are no longer more than 1,6 mm deep.” Checked 2026-09-28.
- US National Highway Traffic Safety Administration, on eCFR: 49 CFR 571.139 — Standard No. 139, New pneumatic radial tires for light vehicles — S3 and S5.4: “Treadwear indicators (TWI) means the projections within the principal grooves designed to give a visual indication of the degrees of wear of the tread”; they show wear “to a tread depth of one sixteenth of an inch”. Checked 2026-09-28.
- Goodyear (UK): How To Read Your Tyre Sidewall Markings — “Tread wear indicators”: “The letters ‘TWI’ show the location of the tyre’s tread wear indicators.” Checked 2026-09-28.
- Pirelli (UK): Tyre Markings: Understanding Writing and Codes — tread depth: Tread wear indicators are “sometimes indicated by the letters TWI but this can vary depending on the manufacturer”. Checked 2026-09-28.