I can fit exactly 16.1 US Gallons into the tank, as I found out today.
(Guess how I found out?)
Rusty J
'80 Westy -- The Mysterious "Smoking Van" from The Vw-Files
"The Fahrvergnugen Is Out There"

Richard A. Jones quoted:
>>>After all, 2WD Vanagons have
>>>~13 gallons of gasoline in the same place.

And then added:
>Hey, it's 15 gallons--at least in the ACs!