If it is indeed the little white gear that is broken (usually it breaks
so that it forms a 'c' instead of an 'o'), you can fix it by taking
apart the dash, removing and *carefully* taking apart the
odo/speedometer.  I say 'carefully' because it is very easy to
accidentally move the speedo needle so that it no longer shows the
right speed.  On the other hand, that might not be all bad with our
speed demon vans ;^).  Put a dollop of glue on the broken gear (but not
on the spindle, of course) and away you go.  Mind you, I've done this on
a '78 VW Scirocco, not an a Bus/Vanagon, but the procedure can't be all
that different, since they're all VDO speedo's.  

Joachim
--
I'm not an engineer, but I play one at work.