Comparing Tire Sizes ST185/80D13 and ST175/80R13 and Bias Ply/Radial Tire Construction

Question:

I have a 1999 Coleman Mesa pop up trailer and I need to replace the tires. I brought the tires, which are ST185/80D13 to a local tire shop and asked to have them replaced I know very little about tires - other than I dont want to have a blow out when I pull the camper 3,000 miles this summer. They put new tires on, but I noticed they are ST175/80R13. I called and asked why they dont match the old tire I brought in and was told that the old tire is antiquated technology and that the new tire is far superior. Im curious what you think?

asked by: Steve

Expert Reply:

We offer both of the trailer tire sizes you mentioned, ST175/80R13 and ST185/80D13. In terms of size they are very close to one another; the ST175/80R13 size measures 24.1-inches in diameter and 6.9-inches wide while the ST185/80D13 size measures 24.4-inches in diameter and 7.2-inches wide. Both tire sizes fit on the same wheel sizes, 13-inchers that are 4-1/2, 5 or 5-1/2-inches wide. In most trailer applications these two tire sizes are going to be interchangeable.

The "antiquated technology" mentioned to you refers to the tire type. The letter D in a tire like Loadstar ST175/80D13 # AM1ST77 denotes that it is a bias ply type tire; the letter R in a tire such as # AM10210 denotes that it is a radial tire. The difference is simply how the belts are arranged inside the tire. Bias ply tires have stiffer sidewalls that make then better suited to rougher off-road driving while radial tires have sidewalls that can flex more which gives better traction and cornering performance on the highway. Both tire types are still made to handle these different needs and it may be unfair to call one antiquated.

What I suggest you do is confirm your pop-up's GVWR, the most it can weigh when loaded. On a single axle Coleman Mesa pop-up each tire needs to be able to handle half that weight. Of course you will ideally have some extra capacity for safety margin. If you locate the weight Load Range rating on your new tires you can refer to the linked article to determine how much weight each can handle.

The best way to avoid tire failures is to always inflate to the maximum psi pressure shown on the tire sidewall, and to operate the tires within their weight capacity and within their speed rating.

expert reply by: Adam R