I towed a tag trailer 450 km down the highway at 55-60 mph and got 37 liters/100 km. (6.4 mpg). Can you get a Dodge or GM to go through that much fuel?
The worst my 97 CTD got was 8.3 running wb on 80 to Laramie at 80+ mph with fiver and strong headwind. The worst my 03 ctd ever got was 8.3 hotfootting it from Phoenix to Flagstaff with the fiver. Craig