I have a tired '85 Westy, and my son has a strong '87 Westy, both with
autos.  With a pretty heavy load, we both get 16-17mpg on the highway at 65
to 70 (flat terrain only for me). Autos are 1:1 top gear, and there is
always some torque converter slip, so they will have poorer mileage than the
manual.

The Vanagon auto is probably about the same as the '65 Mustang, a three
speed hydraulically operated unit.

Modern transmissions have four to eight+ speeds, locking torque converters,
and are controlled by their own computer.  They will get better gas mileage
than a manual transmission in the same car, which is why most manuals have
disappeared (CAFE standards).

All it would take for you manual diehards to wish you had an automatic is
driving for 60 miles (about two hours) in stop and go traffic whenever you
enter or leave Seattle on a trip!

Stuart