I have 342's in my 03 with 34" tires. I'm running lower than the chart says. 70 MPH, I'm around 1900. MMMMM EGT's are very low too. One of the problems with the 3rd gens is they have the aerodynamics of a parachute. There isn't much you can do but here's a couple of trick I used. 4th Gens are 24% more slipperier er er er.
I retrofit a 2nd gen air dam under the factory. You'd never know it's there to look at it. Reduces the air from going under the truck crating friction on all that hardware.
I bushed out the gasket between the door and windshield. It stops the air from being forced into the door frame, causing less drag. It makes the cab quieter too. That's a good sign.
Run weatherstriping around the doors where ever there is a gap air can get into. Especially the front.
I have a roof high cap on the bed. I put some air tabs around the back of the cap breaking off the vacuum on the back window. They work!
I think, the combo of all that is reducing my friction. I'm within 100 RPM of @
F350Cummins truck now. I just recently did the door sealing so I haven't checked the RPMs again. I just know the cab is quieter now which tells me I hit a nerve.