RE: The speedometer and odometer are fed separately.
Actually, they are both fed from the same source, the speedometer cable.
But the odometer is a direct physical connection to the cable.  The
speedometer dial that you can see is driven/moved by induction through a
spinning magnet inside an aluminum cup, consequently the speedometer can
vary, no direct connection.  The odometer is counting the turns of the wheel
in the case of the 2wd Vanagon and the inaccuracy can only come from tire
diameter being different from the factory spec and not turning at 805
revolutions per mile.
I happen to be lucky, my odometer and speedometer are in very close sync,
both read about 2% high based on measured mile courses on the highway.  BTW,
although my GPS seems to provide accurate speed, it ends up way off on
distance, usually many miles low over a couple of hundred miles.