Iirc, the 81 wagon used the transmission from an SD in it, as the 722.318(I think... whatever the 123’s got in 82) and the diff would have no contribution to the speedo at all. However, I am unaware of how the early abs system worked. Amazing to even have that in 81...

If all you changed was the diff, then the speedo would work, but have a reading indicating a slower speed than traveled. If you also changed the speedo... yeah, you are going to have issues, unless you also change the transmission and installed the mechanical cable to drive the newer speedo.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!