ST vs LT Trailer Tires: Which to Use?

What are ST and LT tires?

The letters at the front of a tire size tell you what the tire was designed to do. ST stands for Special Trailer, and LT stands for Light Truck. Both types appear on trailers, but they serve different roles and follow different federal standards.

Under the Federal Motor Vehicle Safety Standards, an ST tire is a tire for use on special-use trailers, which includes ST, FI, and certain small diameter rim tires. An LT tire is a tire designated by its manufacturer as primarily intended for use on lightweight trucks or multipurpose passenger vehicles. That definition comes from the same regulatory sections that govern tire selection for vehicles up to 10,000 pounds gross vehicle weight rating (GVWR).

The key difference is that ST tires are made to sit on a trailer axel and carry load without driving forces, while LT tires are built to handle the acceleration, braking, and cornering of a motor vehicle. This distinction affects casing strength, sidewall stiffness, speed capability, and inflation requirements.

Which federal standards apply?

The U.S. Department of Transportation splits tires into different safety standards. ST tires are covered under Federal Motor Vehicle Safety Standard (FMVSS) No. 119, which applies to tires for vehicles over 10,000 pounds GVWR, specialty tires, and motorcycle tires. The same standard also applies to new pneumatic light truck tires with a tread depth of 18/32 inch or greater, but most LT tires used on passenger vehicles fall under FMVSS No. 139.

FMVSS No. 139 is the standard for radial tires on light vehicles up to 10,000 pounds GVWR. It explicitly excludes ST tires, so an ST tire does not need to meet the higher speed and endurance tests of 139. Instead, ST tires must pass the tests in 119, which include endurance, strength, and high speed performance but are calibrated for the lower speeds typically encountered by trailers.

For trailer installation, FMVSS No. 110 says that a trailer may be equipped with ST tires, FI (farm implement) tires, or tires with a rim diameter code of 12 or below that meet the requirements of FMVSS No. 119. That means you are not legally forced to use ST tires on a trailer, but you must use a tire that meets the correct performance standard for the trailer's GVWR.

Load capacity and load range

Both ST and LT tires come in a range of load ranges, which indicate the maximum air pressure and load the tire can carry. Load range C, D, E, and F are common for LT tires, while ST tires often have load range C, D, or E. The required load range depends on the loaded trailer weight and the number of tires on an axle.

The sum of the maximum load ratings of the tires on an axle must be at least equal to the gross axle weight rating (GAWR) of that axle, as stated in FMVSS No. 120 for vehicles over 10,000 pounds. For vehicles under 10,000 pounds, FMVSS No. 110 has similar requirements. When you select ST or LT tires, always check the load rating at the recommended cold inflation pressure and compare it to your trailer's axle rating.

ST tires often have a higher load rating for a given size because they are designed for stationary or low-speed service where heat generation is low. However, LT tires in the same physical size may have a similar or higher load capacity when inflated to their maximum pressure. The load index printed on the sidewall tells you the maximum load at a specific pressure, so always verify that number before mounting.

Speed rating and driving characteristics

ST tires are generally rated for speeds up to 65 mph (105 km/h), but this is not a federal requirement; it is a manufacturer's recommendation printed on the tire sidewall. LT tires have speed ratings such as Q, R, S, T, H, and higher, which allow sustained highway speeds. If you frequently tow at or above 65 mph, an ST tire may overheat and fail, making an LT tire a safer choice.

The federal standard for ST tires (FMVSS No. 119) includes a high speed performance test, but it is conducted at speeds that reflect trailer service, not the higher speeds of a passenger vehicle. In practice, towing at highway speeds in the 65-75 mph range is common, and many ST tires are built to handle it, but you must confirm the tire's specific speed rating on the sidewall.

LT tires are engineered for the dynamic loads of a moving vehicle, with stronger sidewalls to resist flexing during cornering and braking. This makes them suitable for trailers that are towed at highway speeds for long distances, such as enclosed car haulers or horse trailers. However, LT tires on a trailer must be inflated to the pressure recommended by the trailer manufacturer, not the vehicle manufacturer.

Construction and durability

ST tires typically use a stiffer sidewall and a heavier carcass to resist the constant sidewall flexing that occurs when a trailer is parked or moving slowly. This helps prevent heat buildup and extends tire life in stop-and-go towing. LT tires have a more flexible sidewall for ride comfort and traction, which can lead to more flex on a trailer and potentially higher operating temperatures.

Both ST and LT tires can be radial or bias-ply. Radial tires have cords at roughly 90 degrees to the direction of travel, which is the standard for modern highway use. Bias-ply tires use alternate diagonal plies and are sometimes used on trailers that carry heavy loads at low speeds. The construction type affects the maximum inflation pressure and load capacity, so check the sidewall markings.

A common failure mode for trailer tires is tread separation or blowout due to excessive heat. ST tires are designed to run cooler at moderate speeds, while LT tires may run cooler at high speeds if they have a suitable speed rating. For long highway trips, an LT tire with a speed rating of at least 75 mph is often a better choice than a standard ST tire rated at 65 mph.

When to choose ST tires

Choose ST tires for a dedicated trailer that spends most of its life on the road at moderate speeds. This includes boat trailers, utility trailers, travel trailers, and car dollies. ST tires are optimized for these conditions and generally cost less than LT tires in the same size, though price is not the only factor.

If you rarely exceed 65 mph when towing and your trailer's GVWR is under 10,000 pounds, an ST tire with the correct load range is usually sufficient. The sidewall will be marked with an ST prefix, such as ST205/75R15, and you should always inflate it to the pressure shown on the trailer's tire placard, which may be higher than a passenger car tire.

ST tires also resist flat-spotting better when the trailer sits for long periods, because the stiffer sidewall does not take a set as easily. If your trailer is seasonal, ST tires are a reliable choice. See our best trailer tires guide for examples of ST models that meet FMVSS 119.

When to choose LT tires

Choose LT tires when you need higher speed capability or when you are towing a heavy trailer behind a pickup or SUV. LT tires are designed for vehicles that carry their own weight, so they can handle the additional stresses of towing. Many trailer owners switch to LT tires because they have a higher speed rating and can reduce the risk of blowouts on long interstate trips.

If your trailer has a GVWR over 10,000 pounds, you are already in the realm of FMVSS 119, which applies to both ST and LT tires. However, an LT tire marked with an appropriate load range, such as E or F, may offer a higher maximum inflation pressure and load capacity than an ST of the same diameter. Check the load index on the sidewall to make sure the sum of tire ratings on an axle is at least equal to the GAWR.

LT tires are also a good choice if you use the trailer only occasionally but tow it at highway speeds with a heavy load. They provide a margin of safety at higher speeds, though you must maintain proper inflation. For light truck tires that work well for towing, browse our best light truck tires guide.

Inflation and maintenance

Proper inflation is more critical on a trailer than on a passenger vehicle because a trailer tire carries a constant load and has no driven weight to help it grip. Under-inflated tires flex more, generating heat and increasing the risk of failure. The Department of Energy notes that properly inflated tires improve fuel economy, but for trailers the bigger benefit is safety.

Always inflate trailer tires to the pressure specified on the trailer's certification label or in the owner's manual, not to the maximum pressure stamped on the tire sidewall. That maximum is the pressure for the tire's maximum load, which may be higher than what the trailer axle is rated for. Over-inflation can make the ride harsh and reduce traction.

Check tire pressure when the tires are cold, before you start towing. A drop of just a few psi can overload a tire, especially when you are near its rated capacity. For more on setting pressures, read our tire pressure basics and towing tire pressure adjustment articles.

What to pick for your use

Questions

Can I use LT tires on a trailer?

Yes, you can use LT tires on a trailer as long as they meet the load and speed requirements for your trailer. LT tires are often chosen for higher speed capability, but you must verify that the sum of the load ratings on an axle is at least equal to the GAWR. Many trailers come with ST tires, but LT tires are a legal and safe alternative.

What does ST stand for on a tire?

ST stands for 'Special Trailer.' It is a tire designed specifically for trailer service, with a stiffer sidewall and a lower speed rating than most LT tires. ST tires are covered under FMVSS No. 119 and are commonly used on boat, utility, and travel trailers.

Are ST tires required by law for trailers?

No, ST tires are not required by law, but they must meet the same performance standard as other tires allowed on trailers. FMVSS No. 110 permits trailers to use ST tires, FI tires, or tires with a rim diameter code of 12 or below that meet FMVSS No. 119. You can use LT tires if they also meet that standard.

Do ST tires have a lower speed rating than LT tires?

Generally, yes. Most ST tires are rated for 65 mph (105 km/h), while many LT tires have speed ratings of 75 mph (120 km/h) or higher. Always check the tire's sidewall for its speed symbol. If you frequently tow at highway speeds above 65 mph, an LT tire with a higher rating is recommended.

Which tire type is better for towing safety?

Safety depends on matching the tire to your trailer's load and speed. ST tires are built for trailer service and resist heat at moderate speeds. LT tires can carry more weight at higher speeds when properly inflated. Choose the tire that meets or exceeds your trailer's GAWR and the speeds you plan to tow.

Can I mix ST and LT tires on the same trailer axle?

It is not recommended. Mixing tire types with different speed ratings, load capacities, or construction can lead to uneven handling and premature wear. Always use tires of the same size, load range, and speed rating on an axle, and ideally the same brand and model.

Why do ST tires have a higher recommended pressure than passenger tires?

ST tires are designed to carry heavy loads at a constant rate, requiring higher inflation pressures to prevent sidewall flex and heat buildup. The recommended pressure is listed on the trailer's placard, not on the tire sidewall. Follow the trailer manufacturer's specifications, not the maximum pressure on the tire.

How do I decide between ST and LT for my trailer?

Consider how you use the trailer. If you tow short distances at moderate speeds, ST is likely sufficient. If you take long highway trips at 70 mph or more, or if your trailer is heavily loaded, LT tires provide a higher speed margin. Check load ratings and speed symbols on both types before deciding.