Why am i not getting good MPG?

[th3godf4th3r]

Hey everyone, i posted this over at f150online a couple days too but thought i would post here also

Well i just got home from a 300 mile trip.

anyway about 175 miles of the trip was flat and i had it at cruise control on 65

the other 175 were kinda hilly and going over one mountain pass and had it at cruise on about 75

by the time i pulled into my driveway, my truck computer said 15.2 mpg and my gryphon said 15.1


what im confused about is people like FishChris (on f150online) who said towing his boat he gets 16 mpg.

heres the modifications i have done to it

2008 Ford F-150 Lariat Supercrew 4x4
True Dual magnaflow exhaust with an X pipe
Gryphon programmer with custom tunes
33' BFG A/T's 18" XD monsters
K&N CAI
3.73 gears

City driving which is 95% of the time i only average about 12-12.5 mpg and never really get above that

i drive pretty conservatively too, no rabbit starts. I also do live in Seattle which has alot of hills but i would think i could get better than 12.5

now i didnt buy my truck to get good gas mileage but i just wish i could get some better mpg than i am getting

somebody said with my exhaust i probably lost some low end compression, this would hurt me going up the hills, is that true?

do you think i should try my custom towing tune? any other comments?