Trailer tires have the highest failure rate of any tire category — including passenger and light truck tires. The reasons aren't mysterious. Most trailer tires are underinflated, overloaded, or wildly older than they should be. Here's how to do it right.
Trailer tires are special-built
True trailer tires are marked ST — Special Trailer. They have stiffer sidewalls than passenger tires, are built to handle vertical load with minimal lateral flexing, and are speed-rated lower than passenger tires (usually 65 mph maximum, sometimes lower).
Passenger and light truck tires can be used on small utility trailers but aren't ideal for travel trailers, boat trailers, or anything heavy. ST tires handle the constant load and straight-line trailer dynamics better.
Load index, scaled up
Trailer load indexes work the same way as passenger — a number that maps to pounds-per-tire at max inflation. The catch: trailer tires need to be sized for the trailer's gross vehicle weight rating (GVWR), divided by the number of tires.
Example: a 6,000-lb GVWR boat trailer with two tires needs each tire rated for at least 3,000 lbs. The actual load when fully loaded should never exceed 80% of rated capacity — i.e. 2,400 lbs per tire — to leave a safety margin.
Ply rating
Trailer tires are often described by ply rating (4-ply, 6-ply, 8-ply, etc.) more than by load index. Modern construction doesn't actually use those many literal plies, but the rating maps to load capacity:
- 4-ply (Load Range B) — small utility trailers
- 6-ply (Load Range C) — medium utility, small travel
- 8-ply (Load Range D) — boat trailers, RVs
- 10-ply (Load Range E) — heavy trailers, dual-axle
Why trailer tires fail so often
- Age — trailers sit unused for months at a time, and tires age in the sun.
- Underinflation — owners don't check pressure on a trailer that's been sitting.
- Overloading — trailers are easy to load past their GVWR.
- Speed — exceeding the ST tire's speed rating builds heat fast.
- Flat spotting — sitting unloaded for months deforms the tire's bead seat.
Trailer tire best practices
- Replace at 5 years from DOT code regardless of tread (trailer tires die of age, not wear)
- Check pressure every time you use the trailer
- Inflate to the sidewall maximum (trailer tires are designed to operate at max PSI, unlike passenger)
- Keep speeds at or below the tire's rating (usually 65 mph)
- Use covers when the trailer is stored outdoors
- Move the trailer occasionally — even a few feet — to avoid flat-spotting in storage
If your tow vehicle is a passenger car or light truck
The tires on the tow vehicle need to handle the tongue weight (weight pressing down at the hitch). For most travel trailers, that's 10-15% of trailer weight on the rear axle. Make sure your tow vehicle's tires have load index headroom for that — many drivers max out their tires towing without realizing it.
Trailer tires are the most-neglected and highest-failure-rate category of tire in the world. Don't add to the statistic.