I have an 01 2wd f550 with a 7.3 automatic. Since I bought it two years ago the best millage I have gotten with it is about 7.4 mpg. Does matter how nice I drive it, doesn't matter if I just drive in town vs. hwy. Air filter is good, it doesn't have a cat. converter, i did put a very mild chip in it, and no change. Fuel filter is new, etc. The only thing I can think that might be bad is the muffler might be pluged up but it seems to have full power? I also have a f350 with a 7.3 and that one will get 16-20mpg depending. There is a large difference in gearing. My f550 has 4.88 gears in it which in my opinion is stupid. I think I might regear it to 3.73s. What kind of millege are you guys getting out of the same truck.