I have a 05 6.0 and I can't figure out why every 10,000 miles I loose another 1 MPG. Have taken it to the shop and they keep telling me this is normal since they can't find any codes. Started out getting 15-17 and an now getting 11-12 at 66,000 miles (Hand calculated). I change fuel filters every 10,000 oil every 5,000 and air whenever it needs it. Anybody have any thoughts?