Not to get on your case, but you should have done a little more research before you bought the Ford 6.4, if you were concerned about fuel economy.
There is no such thing as a "stock" tune & better mpg. It just ain't going to happen.
11 to 12 mpg of in-town driving is typical, even with the DPF delete & tuner
I've seen as high as 19 mpg (typically around 17) on the highway with my 2010 F250 4x4 crew cab with the 3.55 gears. I'm running the DPF delete and the Spartan
250 tune. Without the mods, highway MPGs would be 15 to 16.
There are lots of guys on this and other forums claiming "fantastic" mileage increases with the delete & tuner. They are full of baloney!
I did the delete/tuner for 3 reasons:
1. More power for towing & hauling
2. The ability to turn-off, cycle or delete the EGR
With the delete/tuner, the power is fantastic, I can cycle the EGR (which prevents oil contamination from fuel - resulting in longer engine life) and as mentioned, a modest increase in fuel mileage.
If you don't need the extra power, leave your truck "stock", forget the delete/tuner and use the money you'll save to pay for difference in fuel cost, based on the difference in actual mpg versus the small improvement.
If you leave your truck "stock" change your oil every 3,000 miles with conventional oil (not synthetic) because of the fuel contamination issue caused by the EGR. This should hopefully keep your engine healthy for the 10 years your planning to keep the truck.