I've had good results with this company in the past.
http://www.paspeedo.com/index.htm

On 4/21/2011 3:37 PM, Mike wrote:
>  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.
>