Trailer Tire Load Range: The Rating Is Pressure-Dependent

A tire's load capacity is not a property of the tire on its own. It is stated at a specific inflation pressure, and below that pressure the tire carries less.

That single fact is the reason trailer tires fail more often than the tires on the vehicle pulling them, and it is the reason a load range letter on a sidewall is only half a specification.

What load range is

Trailer tires carry a letter, conventionally from B upward, which corresponds to a construction and a maximum inflation pressure, and therefore to a load capacity.

The letters descend from an older ply-rating system, which is why you will see both, and why people talk about a tire being "an eight-ply" when it contains nothing like that number of plies. The letter is the modern expression.

Higher letter means more capacity at a higher pressure.

We are not publishing a table mapping letters to capacities, and the reason is specific rather than cautious. Our retrieval of an authoritative load and inflation table failed today. Load capacity varies by tire size as well as by load range, so a letter alone does not determine a number, and the correct figure for your tire is on its own sidewall and in its own manufacturer's table. Printing a table we have not verified, on a page about the component whose failure ends a trip in a ditch, is not something this site will do.

The pressure half, which is where failures start

Read the sidewall. It states a maximum load AND the pressure at which that load applies. Those two numbers are one specification and they are not separable.

So an underinflated tire is a lower-rated tire. Not slightly less efficient: genuinely rated for less weight. A tire carrying its stated maximum load at less than its stated pressure is overloaded, whatever the sidewall letter says.

And underinflation is what actually destroys trailer tires. A tire below pressure flexes more in the sidewall, flexing generates heat, and heat is what causes a tire to come apart. The failure is a heat failure and the cause is usually pressure.

Three things follow that are worth doing.

Check pressures cold, before every trip. Not warm, not after driving. Pressure rises as a tire heats and a warm reading looks fine.

Check them against the trailer's own placard, which states the pressure for the tires the trailer was specified with. If the tires have been changed to a different size or load range, the placard may no longer apply and the tire's own sidewall and the manufacturer's table govern.

Check the spare. Which is the tire that has been sitting under the trailer for four years and is the one you will need in the worst circumstances.

Why trailer tires have a harder life than truck tires

Five reasons, and they explain the failure rate rather than excusing it.

They are loaded closer to their maximum. A trailer's tires are frequently near their rated capacity by design, where a tow vehicle's tires usually have margin.

They sit still for long periods. A trailer parked for months develops flat spots, and rubber degrades with time and with ultraviolet exposure whether or not it is used.

They are less visible. Nobody looks at a trailer's tires as often as their own, and a trailer tire going down is not felt through the steering wheel.

They scrub in turns. Multi-axle trailers drag their tires sideways in tight turns, which is hard on the tread and on the sidewall.

They carry side loads in sway and in wind. Sway loads the tires sideways in a way straight-line driving does not, and an underinflated tire deforms more under that load, which is one of the ways a tire problem and a stability problem become the same problem.

Age, which is the specification with no letter

Rubber degrades with time. A trailer tire with plenty of tread can be too old to use, and tread depth tells you nothing about it.

Every tire carries a date code on the sidewall, giving the week and year of manufacture. Find yours and know how old the tires are.

We are not printing a maximum age, because tire manufacturers and industry bodies publish guidance and it varies, and none was read today. Your tire manufacturer publishes a recommendation and so does the trailer manufacturer, and those are the sources.

What is worth saying without a number: a trailer whose tires still look fine after many years of light use is a trailer whose tires may be the oldest safety-critical component on it.

ST, LT and P tires

Three categories you will see, and mixing them up is a real mistake.

ST, for special trailer. Designed specifically for trailer service, with stiffer sidewalls for the side loads a trailer sees and no steering or driving duties.

LT, for light truck. Designed to be driven and steered on.

P, for passenger. Car tires.

A tire designed to be driven on and a tire designed to be dragged behind something are engineered for different loads. Whether an LT tire is appropriate on a given trailer is a question for the trailer manufacturer and a tire professional, not a decision to make from a forum, and load capacity is not the only variable.

Never fit passenger car tires to a trailer that specifies ST tires without professional advice, and understand that capacity ratings are derated in some substitutions.

Matching the tire to the load

The chain, in order.

1. Find the trailer's gross vehicle weight rating and its axle ratings. On the trailer's own placard.

2. Find what the trailer actually weighs loaded. Not the brochure figure. A public truck scale gives you real axle weights, and the difference between a dry weight and a real loaded weight is where most of the error lives.

3. Divide across the tires and compare against the tire's rated capacity at its rated pressure.

4. Leave margin. A trailer loaded unevenly puts more on one tire than the arithmetic average, and the tire on the heavy side is the one that fails.

5. If the tires are at or over their rating, the answer is to reduce the load or to consult a tire professional about the correct specification. Not to add pressure beyond the sidewall maximum, which does not raise the rating.

Where you stop

• Never inflate above the maximum pressure stated on the sidewall.
• Never exceed the load stated on the sidewall at the stated pressure.
• Never fit a tire of a different size, load range or type without professional advice. The trailer's specification exists for a reason, and axle and wheel ratings are separate constraints from tire ratings.
• Never repair a trailer tire that has been run flat or has sidewall damage. Heat damage is not visible and a repaired sidewall is not a repair.
• Tire mounting, balancing and wheel work is a tire shop. Bead seating is a real hazard.
• Wheel bearings, brakes and axles are a trailer shop. A hot hub is a bearing problem, not a tire problem, and it is a stop-immediately problem.
• If a tire fails at speed, hold the wheel straight, come off the accelerator and let the combination slow. Do not brake hard and do not steer sharply.

The full weight calculation for the combination is the thing to have done before any of this, because a trailer within its ratings is a trailer whose tires were specified for the load.

The short version

A load range letter is only half a specification: the sidewall states a maximum load at a specific pressure, and below that pressure the tire is rated for less. Underinflation causes heat and heat is what makes trailer tires come apart, which is why cold pressure checks before every trip matter more here than anywhere on the vehicle. Check the date code, because rubber ages regardless of tread. Weigh the loaded trailer at a scale and compare against the tires' actual rating rather than the axle rating. And get the correct specification from the tire's own sidewall and its manufacturer's table, not from any article.