Ryder...I think that your pretty much stuck with the mileage range. Lem is right when he says that it depends on how you drive. I have done a lot of hauling, city driving, some racing and gone for hundreds of miles without comming to a stop sign and I too, have gotten as low as 15 (hauling in the city) and as high as 22(empty on the freeway the whole trip). On average I get 18 to a tank, and that is some town driving, some hauling and some longer trips to town in it. Not too bad if you ask me.
At the same time, the thought of the excursion and better mileage due to the aero dynamics probobly pretty accurate. Do some research, that is really what most of the big rig deisels are getting right now. But I would say that the big gas rigs are not doing that much better. To me, not justifiable to sacrafice the torq and power that deisel engines offer. Not to mention the sound, feel and overall performance you get!