I keep mileage records on all my long trips, and I've always been amazed at
how my best mileage always seems to be in high mountains, esp. in Colorado
where the passes are highest.  This is not a one-time observation, but one
I've seen again and again.  Performance other than mileage is always
terrible- I struggle in third and sometimes second up one pass after
another, often running the heater to keep the van from overheating.  Then,
exhausted from nursing the van up and down mountains all day I fill up and
discover that my gas mileage is 20% higher than normal.

This seems counter to what one would expect.  As others have written, with
thin air one would expect to run rich and therefore get worse performance
(which I see) and worse mileage (which I don't see).  Here is the only
explanation I can come up with:

Perhaps the vanagon AFM responds to more mass of air more than volume, and
so keeps a reasonable mixture?  I've never seen plug fouling, black smoke,
or evidence of rich mixture other than poor performance.  Because the
available quantity of air is limited, horsepower and top speed are low. 
But the low speed also helps mileage via less wind resistance, always an
issue with vanagons.  The thin air also gives less wind resistance.  So
perhaps the lack of wind resistance via thin air and low speeds makes up
for any problems with the mixture.

Comments, flames, etc. are welcome.

Bill
St. Louis, MO
www.geocities.com/yosemite/gorge/1706