In my 07 6.7 liter my overhead was over about 1 mpg. When it said 18 it was 17. That was with the stock 265/70/17 tires. With the 315/70/17 I have on it now, when it says 16 I'm actually 17.2. I have also noticed that it is very dependent on your driving habits. Around town I average between 11 & 13, on the overhead, when I drive it & my wife averages around 15 on the overhead.
On my brothers 07 5.9, he gets 19 all day long & if you watch the overhead it was always between 20 & 23. His is a manual & mine is a auto. He has a cap & I have a tonnue cover now, but didn't have anything for awhile.