While wind, hills, heavy/light foot, weight, speed and shape are major factors, altitude seems to play a part in mileage, as well. This is probably due to the same drag conditions you find causing wind to affect mileage, since higher country has thinner air, thus less drag (aircraft use this fact for efficiency as a matter of course). I find with both my Bounder and with my F-150 that I get better highway mileage in Colorado/Wyoming (when not in the mountains, obviously) than I do when further east -- trips to Michigan, Maryland, etc.
Just for example, on a trip to Michigan not long ago, my mileage in my Bounder ran from 6.8 to 9.0, with reduced values as I got further east (winds played a big part, too, of course). About three years ago I took a trip in my F-150 from Colorado across the UP of Michigan, then up through New England, then back across I-70 from Maryland to Colorado. All else being equal, I got 1-2 mpg better in Colorado and western Kansas/Nebraska than I did further east, running at the same speeds.