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At these prices, I thinking about the 10-ply unless someone thinks this would be a mistake.
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Just be aware that the higher the number of plies the harsher the ride... you also risk losing a tiny bit of traction (less footprint on the ground) and tread cleaning action -- it is the minor flexing of the sidewall that clears mud from the lugs...
(a) I've known people who've put 6-ply R1s on 700-1000 lb machines then pulled them off because they ride so rough -- even with only 6-8 PSI...
(b) Pulling tractors use lower ply ratings so they'll put a larger footprint on the ground, similar to "wrinkle-wall racing slicks" for dragsters...
Some sidewall flex is good, in most circumstances.
Why are you looking for the 10-ply? Puncture resistance?
Another "spec" to consider in addition to ply rating is the max weight rating. For example, ATV tires have VERY low weight ratings because they're designed for lightweight machines. Garden tractor tires typically have much higher weight ratings, and skidsteer or "Ditch Witch" tires even more....
Personally, I think I'd stick to 6 or perhaps 8 ply unless I was really concerned about punctures... and perhaps add some type of good sealant instead. Your mileage may vary...
Here's a couple of very good eBay tire stores, but I'm not sure if they have the really high ply ratings:
Tucker Tire -- he has the original-style Power Trac tires
Rustybore's Tire Store -- has Carlisle Super Lug R1s