I'm from south Texas (about 600 feet above sea level), but when I spent a week in Buena Vista, Co (at levels between 8000 to 11,500 feet above sea level) my average hwy mileage on my 2010 2wd Exterra rose from 21 mpg to 25 mpg! What is it about the high altitude that provided such a noticeable increase in gas mileage? Can that be duplicated with a chip to fool the computer to thinking it's in the mountains again?