Hello.
I'm not an auto expert, but would like to offer my 2 cents anyway.
I remember my dad's old car had a carburator, and whenever we'd drive to
Colorado for vacation (we live in Houston) he'd have to get it adjusted
for the higher climate. Then, upon return, he'd get it adjusted back to
the way it had been.
This was necessary because of the difference in oxygen at the two
levels, but with modern cars having so much controlled by computer, I
don't know of anything you must do yourself.
Newer mass air sensors compensate for the intake of air, and adjust fuel
accordingly.
I drove my 96 XLT to Colorado several years ago, and experienced the
same thing. My mileage increased significantly when I reached the higher
elevations, and stayed that way until I dropped back to sea level.
Sorry I can't suggest anything to improve the situation, but wanted to
share my similar experience.

Randy

We're living in a world that's been pulled over our eyes to blind us
from the truth. Where are you, white rabbit?