Trailer Tire Specifications
The structure of a trailer tire comes in two different forms, bias ply and radial. Each has it's own benefits. Read more about the differences and similarities at our Radial vs. Bias Ply Trailer Tires (What’s The Difference?) information page.
The ply rating on trailer tires typically varies between 2(A) and 10(E). This ply rating is intended to indicate the strength of the tires construction relative to other ply ratings. It doesn't necessarily mean that the tire is manufactured with this specific number of plys or layers. The strength or ply rating of the tire is positively correlated with the load capacity each tire can handle. A higher ply rating results in a stronger tire that can handle a higher load capacity.
The sidewall figure specifies the distance between the tire sidewall's outermost point when inflated. This metric is paramount when determining the amount of clearance you will have horizontally once the tire assembly is installed.
Outside Dia. (In.):
The outside diameter figure specifies the height of the tire when inflated. This metric is paramount when determining the amount of clearance needed vertically once the tire assembly is installed.
Load Capacity (65mph):
Unlike automotive tires, all ST tires are tested and rated at 65MPH. This 65MPH test allows the tire to qualify as a high speed tire. Every ST Trailer Tire you see on the highway goes through this exact same test regardless of tire size or manufacturer. The advertised load capacity on the sidewall of the tire is approved at this speed with proper inflation.
The Max PSI is the maximum amount of air pressure the manufacturer suggests to be used in that particular tire. It is advised not to exceed this number as it can inhibit the structure of your tire and make it unsafe to operate.
Trailer Tire Sizing
The tires on a boat trailer might be labeled: ST175/80D13 C
- *ST - These tires are for a trailer (Specialty Tire for trailer use only)
- *175 - The maximum width of the tire is approximately 175 mm at the widest point
- *80 - Indicates the height of the sidewall is 80% of the width (in this case 140 mm)
- *D - This is a 'Bias' tire (D is for diagonal-the belts run diagonally from bead to bead or at about 45 degree angles to the center line of the tire.
'R' is for 'Radial' - the belts run radially from bead to bead or at 90 degree angles to the center line)
- *13 - This tire fits 13 inch diameter wheels
- *C - The load index (also tires can be labeled with a ply rating, typically a "C" load range is equivalent to a "6 ply" rated tire, "B" is a 4 ply and "D" is a 8 ply)