THey come in either a C or E rating, with 2 ply or 3 ply, respectively, IIRC.
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Well, I did have a set of E rating tires on the VX. I moved them to my wife's Avalanche. I had those tires on the VX for 40K, no problem with the stiffer tires. They would have lasted forever, if I wouldn't have moved them. After over 40k on the VX, still measured 15/32" average (new 18/32".)