I believe another important factor here is rolling resistance due to wheel
bearing and brake adjustment along with wheel alignment.

My '89 Carat is the easiest rolling Vanagon I've driven, and it delivers a
consistent 21-22 mpg (US) @ 65mph. Best ever when it was young and strong
was 24 mpg @ 55mph.

My Syncro has noticeably higher rolling resistance (probably due to the
front differential and viscous coupling) and delivers 3 less mpg . It still
has less rolling resistance than some 2wd versions I've driven (and gets
better mileage)!

--
Angus Gordon

birdworks@gmail.com