Most trailer tire failures come from three things: overload, underinflation and age. All three are visible before the trip if you know where to look. This guide explains the markings, how to choose the right tire and pressure, and when to replace tires that still look fine.
ST, LT and P tires
- ST (Special Trailer) tires are designed only for trailers. They use stiffer sidewalls to resist sway and carry more load than a passenger tire of the same size, but they are not designed for the traction and handling demands of a drive or steer axle.
- LT (Light Truck) tires are built for pickups and vans and are also used on many heavier trailers. When fitted to a trailer, their load capacity is used as rated for single or dual fitment on the sidewall; follow the trailer manufacturer’s recommendation.
- P (passenger) tires are rarely appropriate on trailers. When a P tire is used on a trailer, tire makers typically reduce its load capacity.
Replace tires with the type, size and load range the trailer manufacturer specifies on the VIN plate or tire label, or with a tire of equal or greater load capacity that fits the wheel and clearances.
Reading the sidewall
A trailer tire marked ST225/75R15 LR D means:
- ST: special trailer service.
- 225: section width in millimeters.
- 75: aspect ratio; the sidewall height is 75% of the width.
- R: radial construction (B or D would be bias-ply).
- 15: wheel diameter in inches.
- LR D: load range D, the tire’s strength rating.
The sidewall also lists the maximum load and the maximum cold inflation pressure at which that load applies, for example “Max load 2,540 lb at 65 psi”. Find a tire’s overall diameter and revolutions per mile with thetire size calculator.
Load range
Load range letters describe how much pressure, and therefore load, a tire can handle. The letters replaced the older “ply rating”, which survives as a comparison:
| Load range | Ply rating equivalent | Typical use |
|---|---|---|
| B | 4-ply | Very light utility trailers |
| C | 6-ply | Light utility, small boat and camping trailers |
| D | 8-ply | Many travel trailers and boat trailers |
| E | 10-ply | Heavier travel trailers, fifth wheels, equipment trailers |
| F, G | 12, 14-ply | Heavy fifth wheels and equipment trailers |
Higher load ranges carry more only when inflated to their higher pressure. An LR E tire run at LR C pressure carries roughly LR C load, so check that your wheels are rated for the pressure the tire needs.
How much load each tire carries
The trailer’s tires carry everything that is not on the hitch. Divide that by the number of tires to get the load per tire, then add a margin, because loads are never perfectly even: one side, one axle or a bump always takes more. Many owners aim for tires rated at least 10–20% above the actual load per tire. The trailer tire load calculator does the math with a margin, and the trailer cargo capacity calculator checks the axles and tires together.
Inflation pressure
- Use the pressure on the trailer’s tire label or VIN plate, or the pressure the tire maker’s load table gives for your load. For many ST tires this is the maximum pressure on the sidewall.
- Check cold: before driving, or after the trailer has sat for a few hours. Pressure rises as tires warm up; never let air out of hot tires.
- Check before every trip. Trailer tires lose pressure while parked, and a low tire overheats and fails without any warning from the tow vehicle.
- A tire pressure monitoring system made for trailers alerts you to a slow leak while driving.
Speed
Look for a speed symbol or a maximum speed on the sidewall. Many ST tires have traditionally been designed for a maximum of 65 mph (105 km/h), while others are rated higher. Exceeding the tire’s rated speed builds heat, the main cause of tread separation. Respect the lower of the tire’s rating and the posted limit for towing.
Age and the date code
Tires age even when they aren’t used: rubber dries, sunlight cracks the sidewall and the internal structure weakens. That matters more on trailers, which often have plenty of tread left when they reach the end of their safe life. The DOT code on the sidewall ends with four digits: the first two are the week of manufacture and the last two the year. Many tire and trailer makers recommend replacing trailer tires after about five to six years regardless of tread; follow the recommendation for your tires, and replace earlier if you see cracks, bulges or uneven wear.
Storage and inspection
- Cover tires from sunlight when the trailer is parked for long periods.
- Park on a firm, clean surface; if stored for months, move the trailer occasionally or take weight off the tires.
- Look for sidewall cracks, bulges, cuts, embedded objects and uneven tread wear, which can point to alignment, a bent axle or overload.
- Check lug nut torque before trips and after the first miles on newly mounted wheels.
- Carry a properly inflated spare and the tools to change it.
A walk-around with a pressure gauge takes five minutes. It is part of every good pre-trip towing checklist.
This guide is general information. Your vehicle, hitch, trailer and tire manufacturers’ ratings and instructions always come first, and towing rules vary by state, province and country. See our disclaimer.