Don't get too excited thinking that all these lower RPM's are going to increase fuel economy substantially.
I installed some LT285/70R17 Firestone Destination A/T's and had my speedometer recalibrated at the dealership. 1,350 rpm at 55 mph (68RFE and 3. 73:1 gears) and my fuel economy is still not great... ... I'll give it the benefit of the doubt to increase a little, as I'm only at 5,200 miles or so.
In a nutshell, those 408 ci's w/emissions equipment versus 359 ci's are gonna use more fuel. I often miss my 2003 RAM 2500 Quad Cab CTD w/305*555, but I really love the 68RFE.
Greg