Trailer Tire Guide (ST Tires)

What Tire Makers and Ford Say About Trailer Tires:

• Inflation:Carlisle says to maintain the maximum PSI recommended on the tire sidewall and to check pressure with a quality gauge when tires are cold and in the shade, and that under-inflation is the number one cause of trailer tire failure. Ford's towing guide says underinflated tires get hot and may fail, leading to possible loss of vehicle control, and that overinflated tires may wear unevenly and compromise traction and stopping capability.
• Speed: Carlisle says speed symbols on its trailer tires include 87 mph (N), 75 mph (L) and 65 mph (J, ST tires), that speed ratings are test speeds and not recommended driving speeds, and that it recommends driving no more than 60 mph when towing a trailer.
• Age: Carlisle says remaining tread is not an indicator of tire life, because trailer tires often sit in extended storage under static load, which can break down the carcass in a way that visual inspection cannot detect. NHTSA says some vehicle and tire manufacturers recommend replacing tires that are six to 10 years old regardless of treadwear; Michelin recommends replacing tires in service 10 years or more, including spares, and Bridgestone recommends removing its tires from service after ten years.
• Load capacity: Carlisle says never to exceed the maximum load rating stamped on the tire sidewall or the maximum vehicle load rating, whichever is less. Carlisle also pairs load range C with a 6-ply rating, D with 8-ply and E with 10-ply. The load range to ply rating converter and the ply rating guide list the letters, and the load index chart the numbers.
• Spare: Ford's towing guide says a conventional, identical full-size spare tire is required for trailer towing; mini, compact and dissimilar full-size spares should not be used. The spare tire chart quotes what other manuals say about spares, and the tire age and DOT code decoder covers the age point above.