RE: Cruising Speed
I have no experience with a Bus.  But the following applies to expedience
with two Vanagons.
This was true with both my '85 1.9L 4 speed and also with my '90 automatic.
And these are results that have been repeated on several occasions, not one
time or anecdotal.
If I cruise flat, long freeways at 55 mph my gas mileage will be from 1 1/2
to as much as 3 miles per gallon better than if I'm running at 65 to 70.
Period.
An acquaintance of mine that used to drive a cherry '90 Westy, 4 speed, and
never got above 55, consistently got 21 to 23 mpg.  And she put a lot of
miles on her rig before selling it and going to something bigger.
I also find an improvement, maybe 1/2 mile, using the cruise control.
This is all based on mileage over the same stretch of highway on trips that
I make several time each year.
YMMV