Spirit... is right on....
This is true. Altitude can have a radical effect on fuel consumption. The ECU controlled FI systems in today's engines have excellent compensation built in, like High Altitude compensators that advance ignition timing, etc, but it would not be unusual to see a 3 to 4 HP drop for every 1,000 feet in elevation increase.
It's also true at high elevations, typically higher octane fuel will not compensate for the lack of Oxygen, and actually may "burn" better than 91 octane.
been pumping 87 in a 2004 gx for years, gets about 16.8-17mpg combined...up to 19 for mostly highway