I know this is older post, but here goes anyway. The whole mileage thing is very interesting as a Hemi "MOOSE" owner. I use my truck aka "TANK" for hauling a 4000LB boat now and then and a few times each summer I load up a 2000 lb pop up truck camper on it. Empty around town I usually around 13.5 MPG, It's a 04 QC 4x4 Hemi 3.73.
On a 6k long trip out west this year I got around 11, I feel the wind drag of the camper was mostly to blame, had lotsa wind and of course the high altitudes. When I pull the boat I am in the same ballpark.
When I am pulling or hauling with the 3.73 in Tow/Haul (T/H locks out 5th gear and changes some shift patterns), I find the truck jumps in and out of the torque converter in 4th. Makes me uncomfortable how often it does this. Also you cannot lock this tranny in 3rd where it would be nice to do so, in a headwind, in hilly areas etc. This is why I plan to do the 4.56 gears to bring my rpm's up. There is really only a 200-250 rpm difference between the 4th and 5th gears, I want to pick up at least 400-450 rpms at most speeds. Next year I'll report back and let you know if my mileage tanked, stayed the same, or improved slightly under the different conditions. One shop says my mileage will be 'terrible' with the 4.56. I think empty HWY may drop a bit, but I think I may see increases around town and hauling as it should be less strain on the motor and tranny. I hope the new gears ease the life of the tranny. The TANK may be a bit spunkier too-nice side effect. Any thoughts?
I'll try and get a picture in here soon, my photo file is at home and I am at work now...SHHH..haha