2008 F350 5.4L crew cab, long box 4x4. I've been hand calculating my miles (although the dash calculator has been fairly close) stock (original tires from when i purchased it a couple months ago, aired to i have no idea) i went through several tanks at about 60% freeway 13 overall average mpg. i then installed a 5 star tune (87 econ; also brand new E rated tires to 65 PSI - thinking about messing with tire pressure, but don't know if it is worth the hassle) and i've run 3 tanks through it now, i've had an average of 14.4. when it was a little warmer, i was actually at 15.1 but the last tank really brought me down. I think that my 4x4 hubs aren't fully unlocking (just going by road feel). i've got shocks on the way, going to rebuild the hubs at the same time and see if my suspicion was correct.