I have been driving my '86 Synchro out west for 10 years, up and down from
almost sea level to 8,000+ feet.  Currently I live at 2,200 feet.  The gas
mileage always improves (from about 19 to 21 mpg)above 5,000 feet as well as
the engine running stronger and smoother.  This includes the round trip
element to account for both uphill and downhill.   My old SAAB shows the
same behaviour.  Have checked this over and over.  The engines run much
better at altitude.