Home › Guides › How to Read a Trailer Tire Sidewall — Every Number and Letter Explained

How to Read a Trailer Tire Sidewall — Every Number and Letter Explained

🛒 Shop These Parts at AAA Trailer
ST235/80R16 Taskmaster Provider — 8-Lug Steel WheelBest-selling tire & wheel combo — heavy duty 8-lugShop Now →Taskmaster Rambler 205/75R15 + Vesper 5-Lug Gray WheelBest-selling 3.5K tire & wheel combo — Load Range CShop Now →Taskmaster Trinity Fork Aluminum WheelBest-selling aluminum wheel — 15" machined finishShop Now →Taskmaster Trinity 15x5 Aluminum Wheel — 5 on 4.5Machined aluminum — 3.5K–4.4K axlesShop Now →
📖
Quick Answer

Using ST205/75R15 as an example: ST = Special Trailer, 205 = width in millimeters, 75 = sidewall height is 75% of width, R = radial construction, 15 = fits a 15-inch wheel. Then find the Load Range letter (C/D/E), the max PSI, and the DOT date code. Those four things — size, load range, inflation, and age — determine whether the tire is right for your trailer.

The Size Code Decoded

ST 205 / 75 R 15 101/97M
STTire Type
Special Trailer — for trailer axles only
205Section Width
Width in millimeters at widest point — 205mm ≈ 8.1 inches
75Aspect Ratio
Sidewall height = 75% of width. 75% of 205mm = 154mm tall sidewall
RConstruction
R = Radial (steel belts perpendicular to travel). D = Bias-ply (diagonal belts)
15Rim Diameter
Fits a 15-inch wheel. Must match your wheel exactly.
101/97MLoad Index / Speed
101 = 1,819 lb max load (single). 97 = 1,609 lb (dual). M = 81 mph speed rating

Load Range — The Most Important Marking

Load Range Ply Equivalent Max PSI (typical) Use Case
B 4-ply 35 psi Very light trailers — rare on utility trailers
C 6-ply 50 psi Light utility trailers, small cargo trailers under 5,000 lb GVWR
D 8-ply 65 psi Most common — medium duty trailers, dump trailers, enclosed trailers
E 10-ply 80 psi Heavy duty — high GVWR trailers, severe use, mountain towing

A higher load range tire always has more load capacity than a lower range in the same size. Never downgrade the load range when replacing tires — if your trailer came with Load Range D, replacing with Load Range C reduces your capacity by 25–30% and risks a blowout.

The DOT Date Code — Know Your Tire's Age

Every tire sold in the U.S. has a DOT code on the sidewall. The last 4 digits are the date code:

DOT A1BC D234 2524
25 = Week 25 of the year  |  24 = Year 2024  |  This tire was made in June 2024

The date code may be on the inner sidewall (facing the trailer). If you can't see it from the outside, you may need to look under the trailer or dismount the tire.

Radial vs. Bias-Ply — Which Is Better?

Bias-Ply (D)

Diagonal internal belts

  • Tougher sidewalls — better puncture resistance
  • Lower cost
  • Better for slow-speed, off-pavement, or rough terrain
  • Runs hotter at highway speeds — degrades faster
  • More flex = more heat = shorter life at sustained highway speed

Radial (R)

Perpendicular steel belt construction

  • Runs cooler at highway speeds — longer highway life
  • Better fuel efficiency
  • Preferred for most highway and mixed-use trailers
  • Slightly more expensive
  • Better heat resistance is the main advantage for dump/utility trailers

Frequently Asked Questions

My trailer tires look fine — do I really need to check the date code?
Yes. This is the most common mistake trailer owners make. ST trailer tires age out from the inside. UV, ozone, and heat cycling break down the rubber compounds and belt adhesion long before the tread wears out. A tire with plenty of tread but 7 years old is significantly more likely to fail than a tire with worn tread that's 2 years old. Tread tells you about use. The date code tells you about age.
I'm buying "new" tires. How do I know they're not old?
Check the DOT date code before purchasing. Tires can sit in a warehouse for years before being sold as "new." Industry recommendation is to prefer tires manufactured within the last 12 months. If a dealer won't let you see the date code, that's a red flag.
What does the max PSI stamped on the sidewall mean?
That's the maximum inflation pressure the tire is rated for — and for trailer tires, it's also the target inflation pressure. Unlike vehicle tires where you often run slightly under max, trailer tires should be inflated to the max cold PSI stamped on the sidewall. See our guide on proper trailer tire inflation for details.