I was wondering if anyone here has gone to 19.5 tires. I would like to get my rpm down on the interstate. I already have an NV4500 waiting to go in for the slightly taller overdrive ratio. The tires I'm considering are Cooper RM253 which have a tread that is usable in the mud. One place the are listed as 12 ply the other as 14. I'd probably stay with steel GM 19.5 wheels with simulators since they are available to me. Alcoa's would be nice to reduce unsprung weight, but I doubt they're made in that configuration. The cast wheel options are out since I might get looked at by the DOT at some point.
My main concern is front fender clearance with a 33" tall tire and not giving up much of the already rough ride. I mostly use the truck to haul a gooseneck and might put 5,000 miles on it a year. I figured out I'd drop down to 1900ish rpm at 75 mph with the tire an transmission change.