Well towed my 6x10 enclosed trailer today to job site and back.. 75 miles round trip.. never went over 72mph...................
10.3 mpg
That sounds right on the money when towing. I suspect your low mileage when not towing, and for others who report the same thing, is driving style. It is
very easy to get 13-15 mpg in these trucks but it takes patience and a very conservative driving style to get 18-20. With the 3.92 gears around town though I doubt you'll ever hit 18mpg no matter how you drive. With the 3.21 you might but again you need to drive very conservatively. I read some posters saying they shift at 4000 rpm's and I rarely accelerate the truck to that unless I am merging into heavy traffic and need to get out of the way. That spirited driving will get you in the sub 15 mpg range with ease. it takes a lot of fuel to get these behemoths moving. But hey it's fun so why worry about a few mpg! Enjoy the truck!
I recently did a 700 mile trip from NY to Lower NC and going down traffic was light and I actually hit 20 mpg in the 55 and 60 MPH stretches of HWY. Loved that 33-gallon tank and seeing over 650 miles of range at that mileage when filling up. In the 70-75 MPH stretches it was about 18-18.5. Coming back hit large traffic delays before and after Washington and never exceeded 18.5 MPG the trip back and it was mostly in the 17 mpg range. That 10-15MPH stop-and-go traffic kills mileage. Also, if there is a headwind or tailwind it can make a big difference. Towing last year in my previous truck on one windy day with a strong tailwind I actually hit slightly over 12 MPG while towing my 5000 lb camper on a flat stretch through Kansas. I was also watching my shifts, accelerating slowly, and trying to keep the truck in 8th gear by turning tow mode off (this truck had the 3.21 gears whereas my current one has 3.92). I managed to get that good mileage doing all this but it was too much work and I just started driving normally again the rest of the trip and averaged 9-10 mpg towing over the month-long 8000-mile trip.
I do think if you drove conservatively, tried to keep shifts around the 2000 rpm range and don't let the truck idle for long periods of time you would see a big increase in mileage. Of course, if this is not your driving style it may be detrimental to your mental health
My girlfriend for example drives in sport mode in her car and she DRIVES it! If she had to drive like me she would rather walk lol.