One of the gears in your odometer has started slipping. It is usually the main drive gear for the whole assembly. The two fixes I know of are to remove the gear and scuff up the bar it is installed on so that there is more friction, or judicously place superglue where the gear meets the shaft. I have not tried the former, but got over 150k of troublefree service after trying the latter.

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1985 300 TD 448K
1984 300 TD 278K

1983 240D euro 240k
1994 f-250 idi turbo 330K
1986 f-350 IDI
1987 F-350 IDI

1985 JD 1050 4wd
1965 IH 3660