I noticed when driving my Suburban at higher elevations (I-70 west of Denver in particular) the mileage was a LOT better. I guessed it was because of reduced wind drag... figuring drag is proportional to density, and the air at 5000-7000 is less dense than that at sea level.
At low elevations it used to get 17-17.5 mpg in factory trim (3.42 gears, oe wheels and tires). Now, with different tires (taller, wider, more aggressive tread) and a few other changes (3.73 gears), I'm in the 15 mpg territory.
It's a 1998 L31 4L60E.