Understanding Trailer Tire Number Meaning Made Simple
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.
Breaking Down Trailer Tire Codes
Let's decode a typical trailer tire size: ST225/75R15
ST = Special Trailer
The ST prefix means "Special Trailer." These tires are specifically engineered for trailer applications with:
- Stiffer sidewalls — To resist the lateral forces of trailer sway
- Higher load ratings — Built to carry heavy loads at highway speeds
- Different tread compounds — Optimized for trailer duty cycles
Never substitute passenger car or light truck tires for trailer applications. They're not designed for the unique stresses that trailer tires face.
225 = Tire Width (mm)
The first number is the tire's section width in millimeters. A 225 tire is 225mm wide from sidewall to sidewall. This measurement affects:
- Load capacity
- Road contact patch
- Rolling resistance
- Compatibility with your trailer's wheels
75 = Aspect Ratio
The second number is the aspect ratio — the sidewall height as a percentage of the tire width. For a 225/75R15 tire:
- Tire width: 225mm
- Sidewall height: 75% of 225mm = 168.75mm
Lower aspect ratios (like 60 or 65) have shorter, stiffer sidewalls. Higher ratios (like 80) have taller, more flexible sidewalls.
R = Radial Construction
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.
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15 = Rim Diameter (inches)
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.
Other Common Trailer Tire Prefixes
- ST — Special Trailer (most common for enclosed trailers, boat trailers)
- LT — Light Truck (sometimes used on heavy-duty trailers)
- P — Passenger (never use on trailers)
- M+S — Mud and Snow rated
Load Range and Ply Rating
Trailer tires also carry a load range designation:
- Load Range B — 4-ply, lighter duty
- Load Range C — 6-ply, medium duty
- Load Range D — 8-ply, heavy duty
- Load Range E — 10-ply, extra heavy duty
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).
Why This Matters for Your Safety
Using the wrong tire size or load range can lead to:
- Overloading and tire failure
- Poor handling and trailer sway
- Uneven wear and premature replacement
- Voided warranties
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.
Protect your trailer with intelligent monitoring:
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