After doing the Gotts mod (rest of the truck is stock, other than a K&N), we drove 100 highway miles (70+ mph) and all weekend going back and forth to baseball games (city driving), I averaged 15 mpg. Thrilled with that.
Our Expedition is just our weekend people hauler and my modern day musclecar (guzzler), and for that purpose, I have no problem with 12-15mpg.
If I had to drive this beast daily and feed the fuel tank ... that would be an unacceptable dent in the budget.