I have several posts on another forum regarding my gas mileage. I'll try to copy and paste the ones that concern mileage while towing here. All mileage numbers are hand calculated. My truck doesn't have the DIC thing.
Checked my mileage again this past weekend while towing a 6' X 10' enclosed trailer. A bit over 200 miles each direction - almost all interstate. Running 76 mph - 77 mph into a headwind going I got 8.7 mpg. Coming back I drove 74 mph - 75 mph and had a slight tail wind. Got 9.8 mpg. Not good at all considering how little weight I was hauling. Going to be "interesting" to see what I get when I tie onto my 7' X 16' enclosed trailer....
Mileage from this past weekend. From northwest Louisiana to Lincoln, Nebraska and back. Pulled a 6X12 tandem axle U-Haul trailer to Nebraska. No trailer and empty coming back. Didn't check first tank... Second tank (with trailer) 11.6 mpg; third tank (with trailer) 12.9 mpg (had a 30mph - 35mph tailwind going through Oklahoma and Kansas. ); fourth tank (some with trailer, some in town, and some without trailer) 11.7 mpg; fifth tank (all without trailer) 16.2 mpg; sixth tank (all without trailer, mostly 45mph to 60 mph, and down hill!) 17.7 mpg.