It's been discussed on the list many, many times.

Unit I upgraded my Instrument Cluster to an '85 w/Tachometer, my 84
Westy's speedo was way off too and Bentley confirms it if you look
closely.  In fact my speedo was off by exactly as Matthew's was and I
run 195's as well.  Speedo would read around 80 and the speed
indicator sign on the highway said I was going 59mph.  This is very
typical of 80-84 models.  The 85-later speedometer is so much more
accurate and now I have a tach to monitor the engine more closely.
See my write-up on my site (need Acrobat to read it) if you wanna
retro-fit:

http://www.oldvolkshome.com/late_van_cluster_install.pdf
--
Jim Thompson
84 GL 1.9 "Gloria"
84 Westfalia 2.1 "Ole Putt"
73 K Ghia Coupe "Denise"
72 411 Station Wagon "Pug"
oldvolkshome@gmail.com
http://www.oldvolkshome.com
***********************************
On Nov 16, 2007 12:22 PM, Matthew Snook <matt@snooksband.com> wrote:
> Thanks for the responses, folks.  But now I have to admit that I can't even
> report my own dilemma properly.  I don't know what I was thinking.
>
> The speedometer part _does_ work, but not correctly.  It reports that I'm
> doing eighty when the speed is really about 57 mph.  The odometer, however,
> is stuck @ 243,483 miles - and the trip meter doesn't move, either.
>
> That's the straight scoop, I've got it right this time.  Sorry for the
> brain-lapse.  I'll bet that changes the diagnosis...
>
> Matt
>
>
> On Nov 14, 2007, at 6:51 PM, Matthew Snook wrote:
>
> > My '84 speedometer doesn't turn at all or register a speed.  What
> > is the
> > most common failure?  Is it likely to be the speedo, the cable, or
> > some
> > connection?
> >
> > Thanks,
> > Matt