[17Hatch6MT]

Interesting, the super-high economy in the mountains. I noted this too, on my 2001 Sentra 5MT, 1.8 naturally-aspirated port-injected engine. At home at 600 ft, most tankfuls are 35-40. 40 and better if it's summer and if I'm really trying with 60 mph highway speeds, etc. When on trips in the mountain west, was seeing 45 mpg and better without really trying, iirc.

Guesses: 1. Gasoline may be straight, no ethanol. 2. Mountain driving simulates the pulse-coast strategy, with full-throttle hauls upgrade, and fuel cutoff coasting downgrate. 3. Less wind resistance at altitude 4. Less pumping loss because at altitude, more of the driving is done with wider throttle settings due to the thin atmosphere.

Edit: Guess #5: Higher effective tire pressure as you ascend

 

[17Hatch6MT]

One thing to keep in mind: Tire pressure is really a 'gauge pressure' (relative pressure) unless you measure it with a (rare, lab only I suppose) absolute pressure gauge (and if you do this, add ~14 psi to get the proper or expected reading). If you ascend in the mountains, as the atmospheric pressure drops, the tire pressure increases. Worth about 5 psi at 10,000 ft. relative to sea level.

Yes, tire pressure goes up as temperature goes up. You're supposed to measure at ambient temperature. Yes, you'll have to add air in the wintertime, and take it out in the summer (aside from leaks and slow diffusion), to maintain the desired pressure.