ST (special trailer tire)
ST at the start of a tire size marks a special trailer tire, made for use on trailers only.
Easily confused with
How to read it
Federal rules call them “special tires (ST) for trailers in highway service”. They are left out of FMVSS 139, the standard for car and light truck tires, and covered instead by FMVSS 119, which applies to tires for special-use trailers; FMVSS 110 allows trailers to be fitted with them. Carlisle, a trailer tire maker, reads ST175/80D13C as: ST, a tire for trailer use only; 175, about 175 mm wide at its widest point; 80, a sidewall 80% of the width, here 140 mm; D, diagonal (bias) construction, where R would mean radial; 13, a 13-inch wheel; and C, the Load range (LOAD RANGE E).
Also searched as: ST tire, trailer tire.
What each marking means under the standard that defines it. This page describes markings; it doesn’t advise on choosing, fitting, inflating or replacing tires.
Why trailer tires are separate
Carlisle says trailer tire requirements differ greatly from those of car and light truck tires, and that P and LT tires are not designed for trailer use: their more flexible sidewalls can lead to trailer sway, which the stiffer sidewalls of ST tires are designed to reduce.
Speed ratings on trailer tires
Carlisle says most trailer tires used to be rated at 62 or 65 mph, and that its tires now carry speed symbols from J to N (87 mph). It gives J as 65 mph for its ST tires and 62 mph for its other tires. The standard speed chart puts J at 100 km/h (62 mph) (Bridgestone, Alliance). Carlisle adds that speed ratings are test speeds, not recommended driving speeds. Separately, FMVSS 119 requires a tire limited to 90 km/h (55 mph) or less to say so on the sidewall, in the form “Max speed __ km/h (__ mph)”.
Where the markings are
FMVSS 119 lets special trailer tires carry their markings on only one sidewall, where car and light truck tires carry the main ones on both.
More tire sidewall markings
Browse symbols drawn with curves and waves or letters and numbers.
Sources
- Standard No. 139: New pneumatic radial tires for light vehicles, 49 CFR 571.139 (FMVSS 139), U.S. National Highway Traffic Safety Administration (Electronic Code of Federal Regulations), S2, Application; S5.5
- Tire selection and rims and motor home/recreation vehicle trailer load carrying capacity information for motor vehicles with a GVWR of 4,536 kilograms (10,000 pounds) or less, 49 CFR 571.110 (FMVSS 110), U.S. National Highway Traffic Safety Administration (Electronic Code of Federal Regulations), S4.1(b)(2)
- Standard No. 119: New pneumatic tires for motor vehicles with a GVWR of more than 4,536 kilograms (10,000 pounds), specialty tires, and tires for motorcycles, 49 CFR 571.119 (FMVSS 119), U.S. National Highway Traffic Safety Administration (Electronic Code of Federal Regulations), S3(c); S6.5
- Frequently Asked Questions, Carlisle Transportation Products (The Carlstar Group), FAQ
- Speed and Load Rating Charts Explained, Bridgestone Australia, Tyre speed symbol table
- Reference tables: load index, speed category, Alliance Tire Group databook page, Alliance Tire Group (copy hosted by Axon Tire), Speed category table
Last reviewed September 29, 2026 by Rory Hansen. How we research