Trailer Tire Guide: ST Ratings, Load Range, and Sizing

What makes a tire an ST tire?

ST stands for Special Trailer. These tires are built for trailers rather than for cars or trucks, so their construction and rubber compound are optimized for hauling loads at steady highway speeds. Federal regulations treat them as specialty tires: the National Highway Traffic Safety Administration's Standard No. 119, which covers tires for vehicles with a gross vehicle weight rating above 10,000 pounds, also applies to "special-use trailers (ST, FI and 8-12 rim or lower diameter code)." In contrast, Standard No. 139, the standard for new pneumatic radial tires on light vehicles, explicitly excludes ST tires, meaning they are not required to meet the same high-speed endurance tests that passenger car tires face.

Because ST tires are not held to the same speed and load standards as LT (light truck) tires, they have their own ratings. You cannot simply swap a passenger or LT tire onto a trailer that calls for an ST size. The correct approach is to use tires that are specifically designed for trailer duty, as explained in our guide on ST vs LT trailer tires.

• ST tires are regulated under 49 CFR 571.119, not 571.139.
• They are built to handle trailer loads and lower speed limits.
• Always use ST tires on trailers that specify them.

Reading ST tire markings

The sidewall of an ST tire contains the information you need to select and use it correctly. Look for the size prefix "ST" (for example, ST225/75R15), followed by a load index and speed symbol. The load index is a numeric code that corresponds to the maximum load the tire can carry at the reference inflation pressure. The speed symbol, usually a letter like L or M, indicates the maximum speed the tire can sustain for a prolonged period. A load range letter (such as C, D, or E) also appears on the sidewall, but it is not the same as the load index; the load range is based on ply rating and correlates with the construction strength.

The federal standard defines load rating as "the maximum load a tire is rated to carry for a given inflation pressure." This means the load capacity is always tied to a specific inflation pressure. The sidewall will list a maximum load and a corresponding maximum cold inflation pressure. To get the full load capacity, you must inflate the tire to that pressure. For a deeper explanation of load ranges and index numbers, see our load index rating and load range pages.

Load capacity: matching tires to your trailer

The most important rule when selecting ST tires is to ensure that the tires can handle the weight of your trailer and its cargo. Federal Motor Vehicle Safety Standard 571.110, which applies to trailers with a GVWR of 10,000 pounds or less, requires that "the sum of the maximum load ratings of the tires fitted to an axle shall not be less than the GAWR of the axle system." In plain terms, the two tires on an axle together must be able to support the axle's gross axle weight rating, which is usually printed on the trailer's certification label.

For trailers with a GVWR over 10,000 pounds, Standard 571.120 contains a similar requirement. Even if your trailer is lighter than that, you should use the same logic: compute the actual weight on each tire (the axle weight divided by two, plus a safety margin) and compare it with the tire's load rating at your planned inflation pressure. Do not assume that the tire's maximum load is the same at all pressures; the load rating changes with inflation, as we explain in our load capacity chart.

If you are replacing tires, check the existing tires' size and load range. If you are upgrading to a heavier trailer or increasing cargo, you may need a higher load range. The best trailer tires guide lists options by load range and size.

Speed and pressure: the two numbers that matter

ST tires are not designed for high speeds. Their speed rating, indicated by a letter on the sidewall, is typically lower than what you would find on a passenger car tire. Exceeding that rating can cause heat buildup and lead to failure. For example, a speed symbol of L corresponds to 75 mph, while M means 81 mph. Many trailer tires are rated for 65 mph, so it is essential to check your sidewall before driving at highway speeds. Our speed rating guide explains the letters in detail.

Inflation pressure is equally critical. Under-inflated tires generate extra heat and can overload the sidewall, while over-inflating reduces grip and can make the trailer sway. Always inflate to the pressure that corresponds to your load, not necessarily the maximum printed on the sidewall. Use a reliable gauge and check pressures when the tires are cold. The U.S. Department of Energy notes that properly inflated tires can improve fuel economy by 0.6% on average, and that under-inflation increases rolling resistance. Even though that advice is written for passenger cars, the same physics applies to trailers.

For more on setting the right pressure, see our tire pressure basics and towing tire pressure adjustment articles.

Sizing: choosing the right diameter and width

The tire size is a combination of section width, aspect ratio, construction type, and rim diameter. On an ST tire, the size is stamped on the sidewall (e.g., ST205/75R15). The rim diameter must match the wheel's rim size, and the section width and aspect ratio determine the overall diameter and load capacity. A larger diameter tire will carry more load at a given pressure, but only if the wheel well allows it. Always consult your trailer's original equipment specifications or the load and inflation tables provided by the tire manufacturer.

You should also check the rim width range for the tire. Mounting a tire on a rim that is too narrow or too wide can affect handling and may void the warranty. Our rim width range guide explains how to verify compatibility. If you are considering changing the overall diameter, be aware that it may affect the trailer's ride height and the speedometer reading (if your trailer has one). Read our tire size explained article for a full breakdown of size markings.

• Match the rim diameter exactly to the wheel.
• Use the original size or consult the trailer manufacturer's recommendations.
• Check the load index and speed symbol for your load and speed needs.
• If unsure, choose a higher load range to provide a safety margin.

Bias-ply vs. radial ST tires

ST tires are available in two construction types: bias-ply and radial. Bias-ply tires have plies running diagonally, which gives them a stiffer sidewall and lower cost, but they tend to run hotter at highway speeds. Radial tires have plies running at 90 degrees to the tread, which generates less heat and provides a smoother ride, but they are more expensive. Both are regulated under Standard 571.119, but radial tires are more common for modern trailers because they handle heat better at sustained speeds.

When choosing between the two, consider how fast and how far you typically tow. For long-distance highway towing, radial tires are generally the safer choice. For local, low-speed use, bias-ply tires may be adequate and more budget-friendly. Our bias-ply vs. radial trailer tires page compares the two in more depth.

What to pick for your trailer use

Questions

What does the ST in ST tires stand for?

ST stands for Special Trailer. It identifies tires designed specifically for trailer service, with construction and load/speed ratings that differ from passenger car or light truck tires.

Can I use an LT tire on my trailer instead of an ST tire?

No. LT tires are built for trucks and SUVs, not for trailer axles. Their sidewall construction and load characteristics are different, and trailers that specify ST tires should use only ST tires to remain compliant with FMVSS 119 and to avoid overheating or tire failure.

How do I know the maximum speed of an ST tire?

The speed rating is indicated by a letter on the sidewall, usually near the load index. A speed symbol of L means 75 mph, M means 81 mph, and N means 87 mph. Always stay below that limit, especially when towing.

What is the proper tire pressure for ST tires?

The correct pressure depends on the load you are carrying. Use the load tables from the tire manufacturer, which are based on the load index and inflation pressure. Never exceed the maximum pressure printed on the sidewall. Check pressure when the tires are cold.

Why do ST tires wear out faster than passenger tires?

ST tires are designed to carry heavy loads at steady speeds, which generates more heat and flexing than normal passenger use. This can accelerate wear, especially if the tires are under-inflated or overloaded. Check your load and pressure regularly.