I returned earlier this week from a long trip, odometer says 3,800+ miles.
Driving a '90 Westy with stock engine, automatic tranny, 167,000 on the
original engine, only major work ever done was reseal heads at 88,000. And
a new (yeah, NEW) AFM installed just before the start of the trip. For the
first 2/3 of those miles I was either pushing it to meet a schedule,
freeway and highway speeds 60-70.  And getting as low as 15.5 mpg on one
tank, most segments 16-17 mpg.  Then on the way home no rush and no
schedule, I set the cruise control to 55 mph and cruised in the right lane
no matter what the speed limit was.  I did the last 450 or so miles that
way.  And the mileage for that part of the trip was just a little over 20
mpg!!
This confirms, at least for me, the statement that comes up from time to
time that our Vanagons were engineered during the time that the highway
speed limits were 55.  And that is where the WBX engines like to run.
And seriously YMMV.