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?
I'm sure winter blend fuel didn't help either. I gots some crummy mileage this weekend. My dic mileage is perty close to actual last time I checked it, and 80-82mph @2400-2500 rpms empty, cruise on, winter blend and new heavy mud tires got me 15 mpg zippin across the state. Same truck, same trip in the late summer with summer tires, no heavy winds either time was like 16.5mpg.