I typically get 19; I have had as high as 22, and I have a 4x4.
In the winter, I drop a few MPG, due to more use of the 4x4 (I live in an area that averages several feet of snow per year) and winter blend fuel.
IMO, if you are getting less than 15MPG (highway), you either have very aggressive tires, are in need of a tune up, or perhaps have a pending problem. At least, that has been the experience with my ST. I had never dropped below 15 until my cats went bad.
I checked my mileage last week, and over 150 miles of the tank I was hauling firewood, and got 18.
I would think elevation has got nothing to do with it, since the motor is computer controlled, but it would be interesting to see a mileage chart based on elevation. I live at 6500 feet, and very rarely go below 4200 feet. Most of my driving is above 5000 feet. If I tune my '68 Mustang at home (6500') then drive all day down in the valley (4200') it gets killer gas mileage for a carburated 289.