I just did a 4400 mile round trip (houston-cali-houston) and best tank was 18.4 and no worse than 16.2, thats with 8-9 ppl in it 95% of the time going 70-80 mph, hope that helps.... Oh its a 01 7.3 4X4 with 185k on it....

 

My wife mostly drives the X but when we take it on trips monitor the fuel consuption. Average is 17-18 but one tank out East got 19.8 mpg.

Around town like others 14-16 mpg.

Oh, it's got 240K miles.