Usually I keep my AC set on 67. I just spent a few days in the mountains of Colorado (9300+ ft ASL) and found cooling to be insufficient at 67. Pushed it down a degree to 66 and it was fine. On my way home, coming down the big hill from the Eisenhower tunnel, I started to feel cooler at about 8000 ft ASL and pushed it back up to 67. Just curious about the physics behind this. Lower air pressure = less efficiency?