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E rated tires are far more numerous. C or D rated tires aren't near as common, but they are out there. I'm sure C rated tires would be fine for my uses but I wanted a slightly larger tire, which were all E rated.
You are right and I asked a tire dealer friend of mine why that is. He said there are a lot more sizes and it would be very hard to stock all the different sizes in that many weight ratings. I bought the stock size 18” tires in Toyo AT2 C rated LT tires.
I found that if I were to go to a 20" rim I can get a set of Falken Wildpeak AT3's in 275/60R20 that are SL rated. I may go that route when my Falken LT275/70R18's are done. There's only a .2" difference in height.
I also believe it was the tires that made the back end wander around. The tires had proper air pressure (40psi), but they ended up looking like weak sidewalls. I also loaded it on purpose to see what the springs were going to do. I believe replacement tires will be next on the list.
Time for an update to the lead in on this thread. Earlier this week I loaded up another 700lbs and made sure the rear tires were at (the Tire Manufactures) 50psi, and the rear wandering that I complained about before was not there anymore. It was much more stable with the increased pressures.
However I did distribute about 200lbs in front of the rear axle and around 250lbs on the rear axle and about 250lbs behind the rear axle and the rear end sag was minimal (unlike my earlier loading where the majority of the 600lbs was behind the rear axle).
But I do think I'm going to look into a set of SumoSprings. From comments for other F150 owners that have them, they look like a good solution.
I will update this thread in the near future (Have another 600lbs to move).