I took apart a broken Rabbit speedometer to check out how to access to the
speed indicator cup and drive magnet for the speedometer function.
I found that the magnet can be slid fore and aft on the cable drive shaft to
bring the magnet deeper into/ farther out of the steel cup.
This would vary the eddy current effect, which would allow one to calibrate
the indication on the needle.  A shop that's capable would charge $300 -
$500 to do this for you.
  Mine now indicates 62 MPH when I'm travelling @ 65 MPH (checked by GPS),
so If I rely solely on the speedo, I could get a speeding ticket when I set
my cruise at 65.
I'm actually travelling 3 or 4 mph faster than my needle indication, and
this really bugs me.
I am aware of the designed-in inaccuracy of the older VW speedos.
My wheels are stock steel with tires that are stock-sized Michelin Agilis
load C's all around.
I've searched for info for this specific procedure. but to no avail.  I'd
like to take my Vanagon Speedo apart to see if it's similar to the Rabbit
one in this area.
Does anyone know if I have to disassemble and re-assemble to speedo many
times in trial and error method, or is there a way to mock it all up on the
bench and get it done that way?
If I figure it it all out, I'll post it to the list.

Appreciate any help!
Mike B.