i got my '08 f-150 xlt 5.4 flex fuel 4x4 this past november. it doesnt get the gas meileage that my '06 got. the only differences in the two is the flex fuel engene in the '08 and the rear ends. the '08 has 3:55 gears and the '06 had 3:73 gears. the shift points on the '08 seem very close together. i can be driving at 45 mph and be waiting for it to shift into higear.

 

Its got to be the gears,are we talking city or highway i would think city mpgs would be down a little but you should get it back on the highway.