Hey Fred,
If you know the differential ratio of the donor car it would help a lot.
If it was the same and used approximately the same tire size (outer diameter)
you will be in good shape.

Mine is a wagon with 4.56 gears and it reads about 20% faster than it is
actually going. Not sure if this is really all in the gears, though because
the odometer is within about 2.5% of correct, which is acceptable. I suspect
that I need to open the speedometer and tighten the spring and it will be
fairly close.

The gears come in different colors for different numbers of teeth, but I am
not sure which is right for either 4.1 or 4.56.

When you open up the gear housing, oil comes out, which has deterred me
from investigation. I estimate pretty well, and calculate very well,
so it is not a problem for me but scary for passengers.
(Reads 60 at 50, etc)
--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma