asked by: Steven S. 0. Expert Reply: The difference between the letter D in your ST175/80D13 and the R in the tire size of the Wheel and Tire ST175/80R13 part # AM31985 that you referenced is the tire construction. D stands for bias-ply which is a less preferred tire construction, whereas R stands for radial.Click to see full answer. Similarly, you may ask, what is the difference between r13 and d13 tires?The difference is that D stands for bias-ply and R stands for radial. The main difference between a radial ply and a bias ply tire is how the tire is constructed. A radial tire will normally run cooler than a bias ply tire, especially when the tire is under a load. A tire that runs cooler will last longer.Furthermore, what does St stand for on a tire? Special Trailer Also know, which is better Load Range C or D? The difference between an load range C and load range D tire in the same size is the capacity and psi rating. The D rated tire has a higher capacity. For a ST175/80D13 like # AM1ST77, the capacity is 1,610 pounds at 65 psi. For a C load range in the same size, like # AM1ST76 , the capacity is 1,360 pounds at 50 psi.What does st205 75d14 mean?The tire you have now that is labeled ST205/75D14 is a bias tire because the D inside the tire size means diagonal or bias ply construction. A radial trailer tire will be labeled as ST205/75R14. The R inside the tire size meaning radial construction.