>> 
>> My Question: would any of  you Vanagoneers out there with Subie engines
>> supplying horses for your wagon please inform me about any gas mileage
>> improvements you have experienced?  What engine and what mileage
>> increases did you see when you made the change?

As has already been suggested, join THE group that has ALL THE INFORMATION
about this topic, subaruvanagon.com group. All of this "research" information is 
already published. You SHOULD already know everything you need to know
about the range of 2.1 MPG's.

H6 3.0 Subie with an average 18.5MPG; low is 16.5, high is 21. When I travel mostly
freeway miles at 70, I get 20MPG.
Bob
'87 Syncro Westy