Every number and letter stamped on a trailer tire's sidewall tells you something specific about what that tire can and can't do. The two pieces of information that matter most before any purchase are the load capacity in pounds at a specific PSI, and the DOT approval stamp — without those two confirmed, you're guessing.
The Size Code
Take the MERXENG 4.80-12 as an example. The first number — 4.80 — is the section width in inches (how wide the inflated tire is at its widest point). The second number — 12 — is the rim diameter in inches. Simple enough. The ST prefix used on the ST175/80D13 adds a layer: ST stands for Special Trailer, which means the tire is designed specifically for trailer axles rather than drive or steer axles on a vehicle. Don't put an ST tire on your tow vehicle. Don't put a passenger car tire on your trailer.
On an ST-format tire like the ST175/80D13, the three-number sequence breaks down like this: 175 is the section width in millimeters, 80 is the aspect ratio (the sidewall height as a percentage of the section width), and 13 is the rim diameter in inches. The D in the middle is the construction type — more on that below.
Construction Type: What the D Means
That single letter between the aspect ratio and rim diameter tells you whether the tire is bias-ply (D) or radial (R). All MERXENG trailer tires — the 4.80-12, 4.80-8, and ST175/80D13 — are bias-ply construction. Bias-ply tires have cords that run diagonally across the tire from bead to bead in a crisscross pattern. This gives them stiffer sidewalls, which is actually what you want on a trailer — trailer loads bear down vertically on the tire, and stiffer sidewalls handle that better than the more flexible sidewalls of a radial.
Radial tires (marked R) have cords running perpendicular to the direction of travel. They run cooler at highway speeds and have a slightly longer service life on high-speed, high-mileage applications. But for a utility trailer, a garden cart, or a boat trailer that sees occasional weekend use, bias-ply is the correct and appropriate construction type. The distinction only becomes critical when you're running at sustained highway speeds above 65 mph — which brings us to speed rating.
Speed Rating and Why It's the Number Most Buyers Skip
The speed rating on MERXENG's 4.80-12 and 4.80-8 tires is M. Speed rating M means a maximum of 81 mph. On the ST175/80D13, the rating is J — maximum 62 mph. These aren't arbitrary limits. Bias-ply tires build heat as speed increases, and heat is the primary cause of tire failure. Running a J-rated tire at 70 mph on an interstate isn't a gray area — it's operating outside the tire's design envelope.
Here's the practical rule: if your trailer never gets above 55–60 mph and sees moderate use, a bias-ply Load Range C tire like the MERXENG lineup is entirely appropriate. If you're pulling a loaded boat trailer at 70 mph for 300 miles on a summer day, you need a radial tire with a higher speed rating — and that's a different product category, not a different size of this tire.
Load Range, Ply Rating, and the Capacity Number That Actually Matters
Load Range C means 6-ply rated construction. That's the ply rating (6PR) you'll see stamped on the sidewall of every MERXENG trailer tire in this line. It indicates the tire's structural strength, not literally the number of plies in the casing today — modern tires often achieve equivalent strength with fewer actual plies, but the rating system still uses the traditional 6PR designation for Load Range C.
The load capacity number — 990 lbs per tire on the 4.80-12, 1,360 lbs per tire on the ST175/80D13 — is only valid at the tire's maximum inflation pressure. The 4.80-12 is rated to 990 lbs at 90 PSI. Drop below 90 PSI and you've dropped below the rated load capacity, whether you know it or not. This is why tire pressure is worth checking before every trailer season, not just when a tire looks soft. An underinflated Load Range C tire under full load generates heat faster, wears unevenly, and is more likely to fail.
The DOT Stamp
The DOT number stamped on the sidewall confirms that the tire meets Federal Motor Vehicle Safety Standard 119, the federal standard for trailer tires sold in the US. Every tire in the MERXENG trailer tire lineup carries DOT approval. It's not a marketing claim — it's a federal certification. In a product category where a meaningful portion of budget imports arrive without it, it's worth checking before buying anything.
- Size code: section width × rim diameter (e.g., 4.80-12) or width/aspect ratio/construction/rim (e.g., ST175/80D13)
- Construction: D = bias-ply, R = radial — MERXENG trailer tires are all bias-ply
- Load Range C / 6PR: structural strength rating — all MERXENG trailer tires are Load Range C
- Load capacity: always check the lbs figure at the rated max PSI — 990 lbs at 90 PSI for the 4.80-12 and 4.80-8 5-lug; 1,360 lbs at max PSI for the ST175/80D13
- Speed rating: M = 81 mph max (4.80-12 / 4.80-8); J = 62 mph max (ST175/80D13) — not recommendations, hard limits
- DOT stamp: confirms FMVSS 119 compliance — present on all MERXENG trailer tires
