Although most of my HiHy driving is in the relatively flat lowlands, my best milage is always in the S. Colorado mountains - San Luis Valley region where my son lives. I always get above 30 mpg highway FE regardless of the terraine there, which I assume to be due to the decreased air resistance at this 8,000-12,000 ft altitude.

If I remember correctly, air density decreases about 3% for each 1000 ft of altitude - roughly 20-30% less air resistance at equivalent speeds at the above altitude range compared to sea level. I note that Sr. Kite hails from CO.

Pete