Hey guys, total miles per tank isn't a good way to compare mileage from one driver to the next, because everyone fills their tank at a different point. You've got to calculate the mileage you get per tank for it to be a meaningful comparison from one driver to the next. Even if two people say they fill up "once the light comes on," that still doesn't mean much, because the sensors are not particularly precise from one to the next, and one of those two people may fill up at the first station they see once the light comes on, while the other may run another 20-40 miles before they actually fill up, which will make a pretty significant difference.
With that in mind, I have averaged as low as 13.5 mpg on a tank of gas in the coldest winter with my Yakima cargo box on the roof (running 80-90 mph on the highway up to VT for the weekend), and as high as 18 mpg in the summer with nothing on top and driving backroads all day at 50 mph. Assuming a "tankful" is 21 gallons, you can translate those numbers into a per-tank range of 280 - 380.
That said, I have NEVER gone 380 miles beofre filling up - it is very seldom that I run it down to that point. 19-20 gallons is the average fill-up, and I've only gotten 18 mpg twice.
And yes Raque, once you exceed the "sweet spot" around 50 mph, then your mileage will start dripping noticeably. Wind resistance increases logarithmically (if memory serves) with velocity.