I lifted the following paragraph from another forum. There is a separate link further below for ac tires. Nothing new, most builders already are aware. === Q: What does PR stand for in tire size? Answer: PR= Ply Rating. 8 PR means the tire has the strength equal to a tire with 8 ply's of cord in the body of the tire. The newer term for the same thing is load range. The PR is a hold over from the early days of pneumatic tires, when the only way to make one stronger, or support more weight was to add another layer of cord. Not sure what they used in those days, but let's just say it was cotton. When stronger cords (nylon, rayon, whatever) were developed, they could make a tire with, say 4 layers of cord that would be equal to 8 layers of the older cord material. So that tire would have a 8PR although it only had 4 ply's. Most car and light truck tires now use the load range system to indicate the strength of the tire. Not sure, but I think that all on-highway tires use load range letters to comply with a DOT standard. The short answer is, PR is a measure of the strength, or load carrying capacity of the tire. ======== AOPA link for AC tires: https://www.aopa.org/news-and-media/all-news/2013/april/15/whats-so-special-about-aircraft-tires |