A trailer tire size like ST205/75R14 reads left to right: ST means Special Trailer, 205 is the section width in millimeters, 75 is the aspect ratio (sidewall height as a percentage of width), R means radial construction, and 14 is the wheel diameter in inches. The load range letter on the sidewall, B through E, sets the ply rating and the maximum cold inflation pressure.
What each part of ST205/75R14 means
Take the size apart one segment at a time:
- ST — Special Trailer. The tire is built for trailer service only: stiffer sidewalls to resist sway, and a construction that assumes the axle carries load but does not steer or drive. ST tires are not intended for cars or trucks.
- 205 — section width in millimeters, measured sidewall to sidewall on the specified rim width.
- 75 — aspect ratio. The sidewall height is 75 percent of the section width, so roughly 154 mm here.
- R — radial construction. If you see a D in this position instead, the tire is bias-ply.
- 14 — wheel diameter in inches. The tire only fits a 14-inch wheel.
The same logic applies to any ST size: ST175/80R13, ST225/75R15, ST235/80R16, and so on.
Radial (R) or bias-ply (D)
Radial tires have body plies that run across the tire, with stabilizing belts under the tread. They run cooler at sustained highway speed, wear more evenly, and track straighter behind a tow vehicle. Bias-ply tires have plies laid diagonally, which gives a stiff sidewall at a lower price. They remain common on farm equipment and trailers that mostly move short distances at low speed. For regular highway towing, radial is the standard choice, mainly because heat is what kills trailer tires and radials manage it better.
Load range: what the letter is worth
The load range letter corresponds to a ply rating and a maximum cold inflation pressure. Ply rating is a strength equivalence, not a literal count of fabric layers in a modern tire. Typical values are below, but the sidewall of your specific tire is always the final authority — check it before setting pressure.
| Load range | Ply rating | Typical max pressure (cold) |
|---|---|---|
| B | 4 | 35 psi |
| C | 6 | 50 psi |
| D | 8 | 65 psi |
| E | 10 | 80 psi |
A higher load range in the same size carries more weight, but only at its higher pressure. A Load Range D tire inflated to 50 psi is not carrying its D rating.
Single and dual position ratings
Some tires list two maximum loads on the sidewall: one for single position and a lower one for dual position, where tires are mounted side by side in pairs on the same hub. If your setup runs duals, size the capacity from the dual figure. Note that a tandem-axle trailer with one tire per side on each axle is still single position — dual only means paired tires on one wheel end.
Matching capacity to your trailer
Add up the maximum load of every tire on the ground. That total should exceed the trailer's fully loaded weight with a comfortable margin, since load never spreads perfectly evenly across axles. The wheel matters as much as the tire: each wheel has its own load and inflation rating stamped or listed by the manufacturer, and it must meet or exceed what the tire demands. If you are replacing wheels, confirm fitment with our bolt pattern measuring guide, then compare sizes and ratings in the trailer wheels collection. Matching tires by size and load range are in the trailer tires collection.
FAQ
Can I replace Load Range C tires with Load Range D?
Yes, if the size is the same and your wheels are rated for the higher load and pressure. Inflate to the pressure on the new tire's sidewall, not the old C-range figure, or you give up the extra capacity you paid for.
Which is better for towing, R or D construction?
For highway towing, radial (R). It runs cooler over long distances at speed, which extends tire life and lowers blowout risk. Bias-ply (D) makes sense mainly for low-speed, short-haul, or off-road trailer use.
How do I know what pressure to run?
Read the sidewall. It states the maximum load at a specific cold pressure, for example "max load 1,760 lbs at 50 psi." Trailer tires are generally run at that sidewall pressure, checked cold. Underinflation builds heat, and heat is the leading cause of trailer tire failure.
Can I put regular passenger tires on my trailer?
Use ST tires unless the trailer manufacturer specifies otherwise. Passenger tires have softer sidewalls tuned for ride comfort, which lets a loaded trailer sway more than an ST tire would allow.
Every tire and wheel we sell lists its size and load range in the product specs, ships free, and leaves our Orlando, Florida warehouse — usually a short transit to anywhere in the eastern US.