My 2001 Powerstroke Dually isn’t quite as thrifty. But, after 250,000 miles it will still haul 5 hefty “bubbas” on our biannual hunting trip to the Dakotas and Idaho in relative comfort, although it’s good we like each other . Pack in the longbed topper enough firepower, ammo and supplies for a small army. Hook up the 30 foot Mountaineer behind and I will still get 12-14 mpg except in the mountains. Hmmm .... let me see ... how many wussy Priuses would it take to haul all of that? I rest my case. I would like the “chip” you are using .. I never got around to it.
I have a "Supertuner".
The rear end gearing on the dually is probably what is hurting your mileage. Mine is a '97 short bed crew cab with 315,000 trouble free miles and still going strong. I have a Bigfoot truck camper that I regularly haul, weighing about 2200 lbs, and it still gets about 18 mpg at 70 mph on the highway. With the Supertuner tuning installed I can accellerate up 5%+ grades with the camper on board. Works great out in the Rockies.
The Supertuner plugs into the computer port just below the glove box and changes the tuning depending on what you are wanting to do, i.e., tow, high performance, etc. It also reads the error codes, too. I've heard there are better tuners out now, but I don't know any specifics. It was the best $350 I've spent on this truck, for sure.....