I believe that a lot more often than not, fuel mpg in these trucks depend more on your driving situation and style than "the luck of the draw from the dealers lot". The steady speed distance traveled between stop/starts, the number of start/stops on your daily routine, the speeds you drive. Do you accelerate hard and brake hard versus accelerate moderate and coast to an easy brake.

My empty hwy mpg can vary from about 16.5/17 mpg to 23/24 mpg calculated depending upon what speed I drive, 55/60 or 75/85 and being agressive. I can achieve 18/19 mpg combined city/hwy back and forth to work if I behave, less if i don't. So, I think most of it is the individual and situation. If you feel that you are getting poor mpg...play with your computor to learn about how your truck will perform under different driving conditions. The lie-o-meter can be a good learning tool. Reset it at different times, learn, enjoy, then forget about it and enjoy your beast of a truck.
Diablo Predator Plug:
The Predator does allow me great response, shifting, improved mpg and more. I can play a bit and enjoy the truck and power more on a daily basis and still achieve better mpg than driving without the predator and being mpg conscience.

I am enjoying mine and hope you are enjoying yours!!!

 

Mine is around 17.5 MPG combined, 70% Hwy at 80-85 MPH. I have a Predator and I use 8 oz of Power Service silver in each 10 gal of fuel. Someone said I could get 21 if I drove under 70. Grrrrrr! I can't do it!

 

I use the Power Service Silver as well. I can get the high mpg if I behave and try for a long enough stretch to know that it "IS" achievable but that is about as long as it lasts as it is just against my nature to drive the way I would need to maintain it. Besides, this truck is too big and rides to good to just creep along.