Tacoma Mileage

I am new to this forum, so bear with me. I have a 2001, 2.7L, 5 spd, 4wd extended cab. Love the truck. I live in Iowa at about 700 ft above sea level. On several occasions, I have driven the truck to the mountains of Colorado and have notice that the mileage increases dramatically as I go up in elevation. In Iowa, I get approx 15 mpg around town. Above 8,000 ft I have gotten as high as 24 mpg. I know city driving is lower than highway, but I wouldn't expect the long mountain passes to improve mileage. The air is obviously "thinner" at altitude and probably has something to do with the discrepancy. Wondering if anyone knows a tweak that can be done to improve mileage at lower elevations. Thanks.

 

thats the opposite of what i get mileage wise...
although i have found on long trips that out to the mountains i get really horrible mileage(14-17 mpg) and on the way home it gets up to 20-29 mpg on the 3-8000ft (depending on where i go) that i gain going out and drop coming back on the trip... meaning that the trip equates to 17-24 mpg for the total trip avg.
although it would make sense that as the air gets thinner the ECU compensates by putting in less fuel to keep the air/fuel ratio optimal...however the fact that this means less power produced = higher rpm's negating that bonus...