After a nice conversation with Darryl at 1.650.323.0243 (Palo Alto
Speedometer) I learned a passel of new stuff.

The 1600 revolutions per mile speedo for the syncro is unfortunately NOT
easily adaptable to a an 804(?) 2wd application or the reverse.

IIRC a speedo works on a rotating magnet plate working against an iron disk
spring loaded by a calibrated spring.  Spinning the magnet plate induces a
counter emf in the iron disk which tries to "catch up" with the rotation of
the magnet plate.  Without a restraining device like the spring it would do
so and the two devices would spin merrily along together.  Fortunately for
those with speedometers in which the magnet plate does not physically
contact the iron disk and who have the proper "return spring", the rotation
of the magnets produces a force proportional to the speed of rotation which
deflects the iron disk by an angle proportional to the force.

The spring balances this force providing a "linear" angular deflection vs
magnet rotation angular velocity.  Since the magnet rotation is proportional
to the speed, the deflection can be resolved through the graphic on the
speedometer.  IE 10 degrees = 15mph = 150/6kph, 20 degrees = 30mph and so on
until you reach terminal velocity.  (the fastest speed at which you can
enter characters in to a VT100 without making typing errors)

To use a different face graphic one would need at least a different spring
and perhaps a different plate and disk, depending on the percentage change
of MPH vs cable rotation speed.  The speed wizards at Palo Alto Speedo can
provide such a change but unfortunately it is quite labor intensive and
rivals the price of an inexpensive GPS.

BUT! the odo is a separate device housed in the same instrument and can be
calibrated independently of the speedo to measure distance correctly.  So if
your speedo is good enough but your odo is bitchy, you can fix that.  Give
em a call, the number is above.

Glossary:  emf - electrically magic force, invisible but there.

cheers,

pensioner