ST or LT: Picking Tires for a Travel Trailer

Two travel trailer owners can pull the same model with the same load and one of them will spend the summer replacing blown tires while the other never has a problem. The difference is usually the tires. Not the brand, the type, the load range, and whether they were ever the right choice for the weight of the trailer. A lot of campers leave the factory on the least tire that meets the rating, and a lot of replacements get chosen by size alone.

Here is what the letters mean and how to choose a tire that will finish the trip.

What ST means

ST is Special Trailer. An ST tire is built with stiffer sidewalls to control sway, is rated for the load at the pressure on the sidewall, and is designed on the assumption that it will only ever be a trailer tire, never a steer or drive tire. Traditionally, ST tires carried a lower speed rating than passenger or light truck tires, and older ST tires were often rated for highway speeds that are below what people actually tow at on I-10. Newer ST tires from major makers carry higher speed ratings and stronger construction, and that change matters more than anything else in the choice.

What LT means

LT is Light Truck. An LT tire is built to be a drive and steer tire on a pickup, with a more flexible sidewall for ride and grip, and is rated for higher speeds. LT tires in the same size as an ST tire usually carry less load at the same pressure, because the load rating for a trailer application is derated. Some owners switch to LT tires for the speed rating and the construction quality and accept the load trade-off. It works only if the LT tire still carries the trailer's real weight with margin.

Load range: the number that matters most

• Load range is the letter after the size: C, D, E, and so on. Each step up carries more weight at a higher pressure.
• Weigh the trailer loaded, by axle if you can. Divide by the number of tires on the axle. That is the weight each tire carries, and it needs to be well under the tire's rated load.
• The trailer's placard lists the tire size and pressure the maker specified. That is the minimum, not the ideal. Going up one load range in the same size is often the best change an owner can make.
• Check that the wheel is rated for the higher pressure before going up a load range. Wheels have a maximum pressure stamped on them.

A trailer tire running at or near its rated load in Florida heat has no margin. Margin is what keeps the sidewall from cooking. Buy margin.

Speed rating and heat

A tire's speed rating is also a heat rating. It is the speed the tire can sustain at its rated load without building more heat than it can shed. A trailer tire rated below the speed you actually tow at is running hotter than it was designed to, all day, with the pavement adding to it. This is the mechanism behind the blowouts we described in Trailer Tire Blowouts: Why They Happen and How to Stop Them, and it is why the newer higher-speed-rated ST tires exist. Look for the letter on the sidewall and match it to how you drive.

Age, again

A trailer tire that was the right choice in every respect will still fail if it is old. Trailer tires age out before they wear out because trailers sit. Any tire choice for a camper includes the plan to replace them on age, and the date code on a new tire should be recent. We covered the details in Tire Age.

Matching, pressure, and the spare

All the tires on a tandem-axle trailer should be the same type, size, and load range. Mixing an ST and an LT, or two load ranges, puts different-diameter tires on axles that share the load through the equalizer, and the taller one carries more than its share. Set all of them to the same pressure. Carry a spare that matches, and keep it inflated and covered. A mismatched spare that only comes out on the shoulder is a spare that will be the next failure.

What the crew does

Weigh the question, not just the trailer. Look at the placard, the weight, and the tires that are on it, and put the right tire in the right load range on. New tires for trailers are stocked and mounted on site, in the driveway or on the shoulder. If a blowout has already happened, the crew replaces the tire and checks the mate, the wheel, and what the failure tore out. See tire service for what the truck carries, and RV Tire Blowout: What to Do for the minute after it happens.

Frequently asked questions

Can I put LT tires on a travel trailer?

Yes, if the LT tire's rated load, derated for trailer use where the maker calls for it, still carries the trailer's real weight with margin. Check the load tables, not just the size.

Should I run trailer tires at the max pressure on the sidewall?

ST tires are rated for their maximum load at the sidewall pressure, so running max pressure is usual for a loaded trailer. If the trailer is well under the rated load, the tire maker's inflation table gives a lower figure.

Why did the factory tires blow after two seasons?

Often because they were the minimum tire for the rating, running near their load limit, in heat, at highway speed. Going up a load range and buying a better-rated tire is the usual fix.

Do you stock trailer tires on the service truck?

Common trailer sizes, yes. Tell dispatch the size and load range and the crew brings the right one.

Trailer tire question, or trailer tire on the shoulder? Call Duckett Roadside Repair at (850) 495-0366. Dispatch is answered 24 hours a day across the panhandle.