ST Tires vs LT Tires on a Trailer: The Pressure Rules That Actually Apply

ST tires are built for trailers. They carry more load than LT tires of the same size, but they run hotter and gain more pressure on the road. LT tires are built for trucks: they carry a little less load per size, ride firmer, and run cooler. On a trailer, the right pressure for either type comes from the trailer's placard or from the tire maker's load and inflation table after you weigh the rig. Switching from ST tires to LT tires is fine only when the LT tire still meets the trailer's load requirement and the trailer maker allows it.

That's the short version. The details below explain why the two tire types behave differently, and what that means for setting your TPMS.

What ST and LT Mean on a Tire Sidewall

The letters in front of the size tell you what the tire was designed to do. An ST225/75R15 is a Special Trailer tire, 225 mm wide, with a 75 aspect ratio, radial construction, on a 15-inch wheel. An LT245/75R16 is a Light Truck tire built for pickups, vans, and SUVs that steer, brake, and put power to the ground.

Trailer tires don't do any of that. They roll, carry weight, and get dragged sideways through tight turns. That shapes the whole design.

Sources for the load and design comparison: Barry's Tire Tech on ST construction and the roughly 10% load difference, and retired tire engineer Roger Marble's explanation of why trailer tires gain more pressure.

Why ST Tires Run Hotter Than LT Tires

Here's the trade-off most people miss. They get their extra load rating by working harder, not by being stronger in some magic way.

Roger Marble, who spent over 50 years in tire design and forensic engineering, explains on RVtravel that ST tires support more load relative to their size and inflation than LT tires do. More work means more heat. More heat means a bigger pressure rise on the highway. That's also why the industry originally limited ST tires to 65 mph.

So if you see your trailer tires climbing higher on the TPMS than your truck tires, that's expected. It isn't a sign of a problem by itself.

The Pressure Rules That Actually Apply

The pressure printed on the sidewall is the maximum the tire can hold for its maximum load. It isn't automatically the right number for your trailer. Two sources give you the real answer:

1. The trailer placard. The certification label lists the original tire size and cold inflation pressure the manufacturer set for the trailer's rated capacity.
2. The load and inflation table. Weigh the loaded trailer, divide the axle weight by the number of tires, and look up the pressure that tire needs to carry that load. Add a margin rather than running right at the minimum.

Always set pressure cold, before towing. For a quick reference by tire size, our trailer cold tire pressure chart lists common sizes. To read the load range and maximum pressure on your own tires, see how to read your tire sidewall.

One more check people skip: the wheel. Every trailer wheel has a maximum pressure rating stamped on it. If you move to a higher load range tire that needs more air, make sure the wheel can take it.

Switching From ST Tires to LT Tires: The Load Check

Plenty of owners upgrade to LT tires for a stiffer sidewall and cooler running. It can work. It just has to be done with the numbers in hand.

Because an LT tire carries less load than an ST tire of the same size, a straight size-for-size swap can leave you short on capacity. To stay safe:

• Confirm the trailer manufacturer allows LT tires.
• Compare the LT tire's load rating at the pressure you'll run with the weight each tire must carry.
• If it falls short, step up to a larger size or higher load range, then check fender, suspension, and brake clearance.
• Don't mix ST and LT tires on the same axle.

Upgrading also changes what your monitor should expect, which brings us to TPMS.

What This Means for Your Trailer TPMS Settings

ST tires on a loaded trailer often show a larger warm-up than the tow vehicle's tires. Set your high-pressure alarm with that in mind, or the monitor will cry wolf on every hot afternoon. Spend the first couple of tows watching how high each trailer tire climbs after an hour, then set the high alarm just above that normal range. The low alarm should sit close to your cold set point, so a slow leak shows up before the tire drops below what the load requires.

Temperature tells you more than pressure on a trailer. Tires on the same axle should run at similar temperatures. One running clearly hotter than its partner usually means low pressure, a dragging brake, or a bearing issue.

Valve stems matter too. Barry's Tire Tech points out that rubber valve stems flex under centrifugal force and that screw-on sensors add stress, so use metal valve stems if you run external sensors. Internal sensors avoid the issue entirely. We compare both approaches in our external vs internal TPMS guide.

Our trailer TPMS systems report pressure and temperature on every wheel position, from single-axle campers to triple-axle fifth wheels.

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FAQ

Pick the Tire That Matches the Load

ST tires and LT tires both have a place on trailers. The ST design gives you more load per size. LT tires give you cooler running if the numbers work. Whichever you run, set pressure cold by weight, check the wheel rating, and let your TPMS confirm that each tire is behaving like its neighbor.