Look at any trailer tire and you'll see a string of numbers and letters that looks like a secret code: ST225/75R15 or LT235/80R16. These aren't random — they tell you everything you need to know about the tire's size, load capacity, and intended use.
Understanding trailer tire codes isn't just trivia. It matters for safety, performance, and making sure you buy the right tires for your trailer.
Let's decode a typical trailer tire size: ST225/75R15
The ST prefix means "Special Trailer." These tires are specifically engineered for trailer applications with:
Never substitute passenger car or light truck tires for trailer applications. They're not designed for the unique stresses that trailer tires face.
The first number is the tire's section width in millimeters. A 225 tire is 225mm wide from sidewall to sidewall. This measurement affects:
The second number is the aspect ratio — the sidewall height as a percentage of the tire width. For a 225/75R15 tire:
Lower aspect ratios (like 60 or 65) have shorter, stiffer sidewalls. Higher ratios (like 80) have taller, more flexible sidewalls.
R means radial construction — the tire's internal ply cords run radially across the tire. Almost all modern trailer tires are radial. You may occasionally see D for diagonal/bias-ply construction on older or specialty trailers.
The final number is the wheel rim diameter in inches. A 15 means the tire fits a 15-inch wheel. This must match your trailer's wheels exactly.
Trailer tires also carry a load range designation:
Higher load ranges handle more weight but may provide a firmer ride. Match the load range to your trailer's GVWR (Gross Vehicle Weight Rating).
Using the wrong tire size or load range can lead to:
A monitoring system like the TWD-1500 helps you catch tire problems early — pressure loss, overheating, and developing issues that could lead to catastrophic failure.
