Since the pot metal gear is worn out and the shaft comes out easily...The pot metal gear should spin relatively easily (assuming the shaft is in place)(i.e by poking it with a paper-clip).

Here is a video demonstrating this behavior:
https://www.youtube.com/watch?v=wFka_Oi1P7g

If it is not spinning freely than either:
1) The intermediate gear (between it and the tripometer) is jammed up
2) The tripometer gears are jammed up somehow.

Best guess.

I believe the gear ratio difference would rely on the rear differential being different between the two cars. That would explain your differences in MPG.

1977–1983, 240D, OM616.912, 3.69, 1.1 Liter
1977–1985, 300D, OM617.912, 3.46, 1.1 Liter

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