First thing you need to do is put on your flame resistant suit for the inevitable 'If you have to think about gas mileage you shouldn't own a Raptor...'
Second, there are some people on here who get just ridiculous mileage that I cannot come close to (I'm all city driving for the most part) but I'm sure there is some good info in the longer threads if I remember correctly.
I did notice that when I tried the suggestion of putting the truck in manual, locking it in 6th and setting the cruise my mileage was pretty good! I think it was over 13 for that trip IIRC.
Good luck
~BGM