Trailer Tires Explained: ST vs. LT, Load Range, Speed and Age

What ST means and why trailer tires are different

ST stands for Special Trailer. The U.S. Tire Manufacturers Association (USTMA) spells it out as “Special Tire for Trailers” and describes these tires as designed specifically for trailer use in highway service. You’ll see it at the front of the size, as in ST225/75R15 Load Range D: 225 is the section width in millimeters, 75 is the aspect ratio, R means radial construction, 15 is the rim diameter in inches and D is the load range. A “D” in place of the “R” marks a bias-ply (diagonal) tire. Trailer tires are made in both constructions.

A trailer’s tires have a different job from the tires on your truck. They carry weight and follow the tow vehicle rather than steering or driving it. One trailer-tire maker explains that ST tires use stiffer, heavy-duty sidewalls designed to control and reduce sway, and that the more flexible sidewalls of passenger and light truck tires can cause trailer sway problems.

Start with the trailer’s certification label

Federal rules require a trailer’s certification label on the forward half of the left side, where it can be read from outside. It shows the vehicle identification number (VIN), the gross vehicle weight rating (GVWR) and the gross axle weight rating (GAWR) for each axle. The tire and rim sizes and the recommended cold inflation pressure are on that label or on a separate tire placard. Travel trailers often carry several labels, including a tire placard and a cargo capacity label.

USTMA’s RV guidance is to replace tires with the size and load range shown on the certification label. If you’re considering a different size, ask the trailer manufacturer or a tire professional first, because the wrong size or load range can lead to tire failure. Once you have the numbers, shop trailer tires by that exact size.

Load range, weight and margin

ST tires carry a load range letter, which USTMA says identifies the tire’s load and inflation limits. The sidewall also lists the tire’s maximum load and the cold pressure needed to carry it. One trailer-tire maker gives the ply equivalents as B = 4-ply, C = 6-ply, D = 8-ply and E = 10-ply.

Three checks keep the numbers honest:

• Axle ratings. USTMA explains that, by regulation, the tire load rating times the number of tires on an axle must equal or exceed that axle’s GAWR. Fitting tires with more capacity doesn’t raise the GAWR, because the lowest-rated part on the axle, such as the wheels, sets it.
• Actual weight. Weigh the loaded trailer on a certified public scale. USTMA’s method: weigh the whole trailer unhitched, then weigh it hitched with only the trailer on the scale. The second number is what the trailer tires carry. The difference between the two is tongue weight, which the tow vehicle’s tires carry.
• Side-to-side balance. An axle can be within its rating while one side is overloaded. USTMA recommends weighing each wheel position and moving cargo if one is too heavy.

Leave some margin. USTMA notes that a tire reaches its maximum load capacity only at the maximum pressure molded on the sidewall, and that an underinflated tire can’t properly carry its load. If you move up a load range for more capacity, check the wheels too, since USTMA warns that rims may not be rated for the higher load or pressure. The tow vehicle’s tires carry the tongue weight, so they count as well; if your pickup calls for LT sizes, you’ll find them under light truck tires.

Inflation: follow the tire maker

One trailer-tire maker’s instruction is to always keep the pressure at the maximum psi shown on the sidewall, checked with a good gauge when the tires are cold and in the shade. Other makers publish their own load and inflation guidance, so follow the instructions for the tires on your trailer. Two limits hold either way: USTMA says never to go below the pressure on the placard or certification label, and never above the maximum on the sidewall.

USTMA’s RV guidance is to check cold tires with an accurate gauge at least once a month, before each trip and each morning of a trip. Its figures for passenger and light truck tires are a loss of 1 to 2 psi a month, plus 1 to 2 psi for every 10°F drop in temperature, so a trailer that has been parked since summer needs a pressure check before a fall trip. For pressures above 65 psi, USTMA says a high-pressure valve must be used, so confirm that with the shop when new tires are mounted.

Speed ratings on trailer tires

One trailer-tire maker says most trailer tires are rated for 65 mph, and that the default rating for a trailer tire with no speed symbol is 62 mph. Other ST tires carry a speed symbol, the same way car tires do. On a standard speed chart, L means 75 mph, M means 81 mph and N means 87 mph.

Read your sidewall before a long tow. If there’s no speed symbol, USTMA’s advice is to ask the tire maker for the tire’s speed capability rather than guess. A speed rating is a tested limit, not a target: USTMA notes that it doesn’t mean a vehicle can be driven safely at that speed, particularly in bad weather or on poor roads.

ST or LT: can you put light truck tires on a trailer?

Go by the certification label. If it lists ST tires, stay with ST. USTMA’s RV guidance is direct: ST tires have different load capacity characteristics, so they should not be replaced with any other type of tire. The trailer-tire maker quoted above adds that passenger and LT tires are not designed for trailer use. If the label lists an LT size, the trailer maker chose LT tires, and the replacement should be that LT size and load range.

Any change from what the label lists, in either direction, is a question for the trailer manufacturer or a tire professional before you buy. Whatever the type, keep each axle matched. USTMA recommends matching tire size designations in pairs on an axle, and because the ST or LT and the R or D are part of the designation, that also rules out mixing types or bias and radial tires on one axle.

Sun, storage and tire age

Trailers spend most of their lives parked, and that is hard on tires. Ozone and heat from sunlight cause weather checking, a pattern of fine cracks. USTMA says tires with severe weather checking, or any crack that reaches the casing cords, should be taken out of service, and that cracking on RV tires is typically caused by underinflation, overloading, improper storage or improper long-term parking. Look over the tread and both sidewalls before each trip.

For storage, USTMA recommends:

• An opaque, waterproof cover with some vent openings, so it doesn’t trap heat and moisture.
• Taking the weight off the tires if you can, or at least unloading the trailer.
• Keeping tires at the recommended pressure on a firm, level, well-drained surface, and moving the trailer every three months so a different section of each tire flexes.
• Keeping tires away from electric motors, generators and other ozone sources.

Then check age. The last four digits of the DOT code give the week and year the tire was made, so 2224 means the 22nd week of 2024. USTMA says a tire’s service life can’t be predicted from calendar age, and it advises following any service-life recommendation from the tire or vehicle maker. Those recommendations vary. One major tire maker calls for a yearly professional inspection after five years of service and replacement ten years after the manufacture date, whatever the tread looks like. A trailer-tire maker warns that remaining tread is not a sign of remaining life, because trailers often sit under a static load for long periods and the internal structure can break down without any visible sign. Ask the maker of your tires what it recommends, and have older tires inspected before a long trip.

When it’s time to replace a set, Simolary offers free shipping to the contiguous US, processes orders in 1–2 business days (Monday through Friday) and emails the tracking number once your order ships. You can follow it on the Order Tracking page.