While not necessarily an industry standard, they do mean something. Discount Tire uses the C1, C2, D1, D2, etc. as indicators of the max load carrying psi. The number 1 is higher pressure, and 2 is lower.
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Just looks like a load of crap from Discount to me.
"Ply Ratings" mean nothing, because no tire actually has those numbers of plies; they are only old terminology for what is now called the Load Range, and in their table correspond directly to the real (no "1" or "2") Load Range.
In a size and type for which Load Range D tires have their maximum capacity at an inflation pressure of 65
PSI, every Load Range D tire of the same size and type will have the same inflation pressure, so there will be no "D2" tires, and the extra digit doesn't help in comparing tires. It is true that the maximum inflation pressure for the same Load Range varies by tire type, but there is no point in trying to build that into some extended scale, and if an intelligent person were doing that they would just add the pressure: C
50 for Load Range C with a maximum inflation pressure of 50
PSI, and so on.
Try to find two ST205/75R15 Load Range C tires, one with a maximum inflation pressure of 50
PSI (Discount's "C1") and one with a a maximum inflation pressure of 35
PSI (Discount's "C2"). Nope, there's no "C2" - they all have 50
PSI maximum inflation. Why make life more complicated than necessary?
I actually like the way they state it, because you can tell at a glance not just the load rating, but the tire pressure used for max load.
Okay... my motorhome tires are load range with a maximum inflation pressure of 110
PSI. Is that G1? G2? G3? G0? And if there was a number after G, how would you know the corresponding pressure? Memorizing Discount's "special" table seems pointless.