ST Trailer Tire Markings and How They Differ From LT

What ST means on a trailer tire

ST stands for Special Trailer. When those letters appear in a tire's size designation, the tire is designated for trailer service, such as boat, utility and cargo trailers. LT stands for Light Truck, a designation intended for vehicles such as pickups, vans and SUVs. Both markings tell you the service category the tire was built and labeled for. Neither one alone tells you whether a particular tire is approved for a particular trailer. That answer comes from the tire manufacturer's application information and the trailer manufacturer's original specification.

A common piece of forum advice holds that a tire is not a trailer tire unless it says ST. The first part is accurate in a narrow sense: the ST marking is how a tire identifies itself as a Special Trailer designation, so a tire without it is not labeled that way. Whether a tire with a different designation, such as LT, may be used on a given trailer is a separate question. It depends on the specific tire, the specific trailer and the documents that cover both, not on a general rule.

How the service designation works

The letters at the start of a tire size act as a label for the type of service the size is rated under. The numbers and letters that follow describe the tire's dimensions and construction. Because the prefix and the dimensions are read together, two tires can share the same width, aspect ratio and rim diameter while carrying different service designations. That difference matters because load and service information is published for each designation separately, so you cannot assume figures from one category carry over to the other.

Illustrative example of how the designations are written, using commonly seen sizes rather than a specific product:

ST205/75R15
││ │   │ │ └─ 15  = rim diameter in inches
││ │   │ └─── R   = radial construction
││ │   └───── 75  = aspect ratio (sidewall height as % of width)
││ └───────── 205 = section width in millimeters
└┴─────────── ST  = Special Trailer service designation

LT235/85R16
└┴─────────── LT  = Light Truck service designation

The practical effect is that the prefix is the first thing to read and the first thing to write down. If you record only the numbers, you lose the part of the marking that tells you which set of manufacturer information applies.

Reading and recording the marking on the sidewall

This applies to new tires, used tires and tires already mounted on a trailer. For a used tire, the sidewall marking is often the only reliable way to identify it, because the seller's description may not match what is molded into the rubber.

1. Clean the sidewall enough to read the raised lettering. If the trailer is on the ground, make sure it is parked on level ground, the tow vehicle or coupler is secured and the wheels are chocked before you kneel beside it.
2. Find the size designation. It is usually the largest group of letters and numbers on the sidewall.
3. Copy the complete designation exactly as molded, including the prefix. Write ST or LT as you see it before recording anything else.
4. Record the other service markings near the size, such as the load range letter and any load and inflation figures molded into the sidewall, without converting or estimating them.
5. Record the tire manufacturer and model name.
6. Record the DOT code. Its final digits indicate the week and year of manufacture, which is useful when evaluating a used tire.
7. Check the opposite sidewall. Some markings appear on only one side, and the full DOT code may be on the side facing the trailer.
8. Compare what you recorded against the trailer's own label or documentation showing its original tire specification.
9. Completion check: you should have the full designation with its prefix, the tire make and model, the DOT code and the trailer's original specification written in one place.

Stop and have the tire inspected by a tire professional if the sidewall is too worn, cracked or damaged to read, or if any visible damage such as bulges, cuts or exposed cords is present. A tire whose markings cannot be confirmed should not be assumed to match any designation.

An identification worksheet you can use

Keeping each tire's details in a simple table makes it easier to compare tires and to bring the right information to a dealer or manufacturer. The example row below is illustrative only.

Complete sidewall designation	Observed prefix	Expanded service term	Tire manufacturer and model	Trailer model and year	Document reference
ST205/75R15 (illustrative)	ST	Special Trailer	As molded on sidewall	As shown on trailer label	Trailer manual page or tire spec sheet used

ST versus LT: what to check before comparing applications

Identifying the designation and deciding whether a tire suits a trailer are two different steps. Keep them separate so a correct identification is not mistaken for an approval.

1. Identify the designation on each tire involved: ST, LT or another prefix.
2. Locate the exact tire manufacturer's application information for that tire model and size.
3. Locate the trailer manufacturer's original tire specification for your trailer model and year.
4. Write down each instruction along with what it covers, such as a specific size, load range or trailer type.
5. If you are weighing a restriction or a proposed substitution, find the applicable standard or regulation for your jurisdiction before drawing a conclusion.
6. If the documents do not clearly cover your combination, ask the tire manufacturer, the trailer manufacturer or a qualified tire dealer rather than relying on general rules.

Some trailer manufacturers publish broad guidance favoring ST tires on their trailers. Venture, a boat trailer manufacturer, describes ST tires as the designation for boat, utility and cargo trailer service and recommends checking the sidewall for the ST marking. Guidance like this applies to the manufacturer's own products and the tires it specifies. It does not establish load figures, speed limits or interchange rules for a tire or trailer it did not cover.