I love the mileage wars!
Seriously though in my '08 Duramax I have seen 22+ MPG. I have seen < 10 MPG. I have religiously tracked my fuel mileage on paper (OK on my phone ) for 60,000 miles and the actual average is about 14+ MPG overall. That includes city, highway, idling, towing and all.
My guess in the end, no one is really seeing much better over the long run (exceptions apply of course).
I traded up from a Colorado, I got on average 12 MPG, even at +2 MPG (my 14+mpg diesel) more it is cheaper in fuel to run the diesel though.
Its not cheaper to run it when you look at the big picture. Look at the brakes, tires, and oil changes along with other maintenance like Fuel Filters and Air Filters versus your other truck.
I am a two truck guy also. I have a 06 F-350 Diesel SRW 4x4 with 3.73 gears. It has 4" exhaust, Quadzilla Tuner, and a MAC intake. I also have a 2008 Nissan Titan 4x4 with 4.10 gears, 2" lift, and 35" tires. Like you I track all my fuel on my phone for over a year now. The Tires are about $1000 for the Titan and last 50,000 miles. The tires for the F-350 are $1400 and last about 30,000 miles. Brakes are about a wash. Both have washable air filters but thats where it ends. The F-350 takes 15QTS of oil every 5,000 miles. The Titan takes 6.5 QTS every 5,000 miles. The oil filter for the F-350 is $25 and the Titan is $5. The Titan needs nothing else. The F-350 gets new fuel and tranny filters at 20,000 mile intervals but the Titan needs nothing.
To date the Nissan has had the rear axle leaking and a seal was replace, Dana 44. $45 done by me. It also had a tail light go out $2 done by me. The F-350 has needed nothing. $0
As for MPG. Here is what I get. F-350 has 100,000 miles. Titan has 62,000 miles.
F-350: Best tank 23.6mpg. Average around town 17mpg. Average tank to tank is 15mpg. Lowest, 13mpg.
Titan: Best tank 16.5mpg. Average around town is 14mpg. Average tank to tank is 13.7 mpg. Lowest, 8.1 mpg.
Both trucks do no idling. Both are garage stored, so start and go. They both do lots of work. Towing is about 1/3 of the miles for each truck. Biggest load for the Titan is 10,000#, average load is 9,700#. Biggest load for the F-350 is 25,000#. Average load is 16,000#.
Chris