I have a 97 F-150 4x4.It has after market exhaust,K&N air filter and I`m running 2.65 x 16 all terrain tires.No matter what I do I`m unable to get anymore than eleven or twelve mpg.Do you have any suggestions?  

city driving?
with or without tonneau?
proper tire pressure?
it it running properly?  

May be tune up time. If those tires are much bigger than the factory ones, a regearing of your rearend will bring it back to stock figures. Being 4x4 this will also have to be performed on your front axle too. You could get away from doing both axles at once as long as you don't use the 4x4 until both axle ratios are corrected and matching. Plan on replacing your transfer case if you don't.

If your truck is injected and has a computer, try using the power programmer from hypertech. It will recalibrate your computer for the increase in tire size, performance mods already performed by answering yes/no questions when prompted. It costs about $400+/- $50 and is cheap compared to roughly the same per axle alone. Good Luck!  

PRIUS TIME OHHHHHHHH  

You said the P word!! YUK!  

what? P as in Punch-Prius-owners-in-the-face? hahaha yeah i did.

how bout a nice v8 in a prius? someone do it...

i like to mess with prius drivers... i hit a switch next to one, at like a zillion PSI and she totally freaked out. "WHAT JUST HAPPENED TO MY PRUIS? IT JUST MADE A FUNNY SOUND. THAT WAS ABSURD!"  

what engine is in the F-150? The 4.6L and 5.4L aren't known for being the most efficient powerplants on earth.