Being the day after Turkey Day, I figure everyone is sluggish after stuffing themselves, so here is some sluggish reading that I posted in another forum that Mtnguy and I frequent about what a ply rating is:
We have to understand the tire terminology of LT(Light Truck) and ST(Special Trailer) tires, along with the manufacturing history of them to know what we are reading in what is a "Load Range" aka "Ply Rating."
These tires have what is called a Load range or Ply rating. It is just that, a rating, and is not indicative of the true actual numbers of plies in the tire. Modern tires have less true plies than the ply rating suggests. You can always read near the bead of a given tire and see how many true plies are in the tire (sidewall and tread).
OK, you're probably asking yourself, why a tire has less true plies than the ply "rating" indicates? Why not just put the true number of plies on the label? It has to do with how tires used to be made, or more precise, what they used to be made of. Tires all used to have their internal carcass plies made up of cotton cords. When tires were all made up of cotton cords, then the true number of cotton cords inside the tire, was the true number of plies listed on the tire. A 6 ply cotton corded tire had 6 actual plies of cotton cord...and so on, so on.
As technology has progressed, so has the stuff you can now make a tires internal cords from. You now have nylon, rayon, kevlar, and other polymers that are stronger than cotton. Different tire manufacturers will use different polymers to make their tires internal cords from, all of which are stronger than the previous cotton cords.
To keep things uniform, and where you and I can understand it, you have the ply rating come into existence. Take for example Brand X tires. Their tire is made from nylon plies, which 2 plies of nylon might equal 6 plies of cotton. Their tire is thus equivalent to a 6 ply "cotton corded" tire, but only has 2 plies of true nylon cord.
Now Brand Y tires have kevlar in theirs which 1 ply of it, might be equal in strength to 6 plies of cotton. They also have a tire equivalent in strength to a 6 ply cotton corded tire, but it has only 1 true ply of kevlar.
The ply rating, is a rating based off of the old cotton plies strengths. A 6, 8, 10, or higher rated tire has the same equivalent strength of a tire that had that many actual cotton plies..........even though it doesn't have that many actual plies now.
Since there are so many different things that each tire can now be made of in it's ply construction (besides cotton), by using the Ply Rating system, it keeps it uniform across all the brand lines. As a consumer, you aren't really interested in WHAT a tire has inside, as much as you are needing to know HOW much weight you can carry on it.
Each tire manufacturer uses the old cotton corded strengths as a measuring standard to determine their tires strengths with whatever material they use internally for their plies.
Here are the most common Ply Ratings or Load Ranges
C = 6 ply rated...equal to 6 plies of cotton cord
D = 8 ply rated...equal to 8 plies of cotton cord
E = 10 ply rated...equal to 10 plies of cotton cord
F = 12 ply rated...equal to 12 plies of cotton cord
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