yes, the speedo's are model specific for the 123 chassis.
the 240 speedo's ALL are geared for a 3.69 rear diff.
the 300 non turbo's are all geared for 3.46 (unless it's a manual trans, then I don't know)
the 300 Turbo's up to 1984 are geared for a 3.07 diff,
in 85 the turbo's got the 2.88 Diff.
the one anomaly is the 81 Wagon. it's got a electronic driven speedometer, and is only useful on that vehicle.

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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!