Sorry to create confusion with my "main gear" reference. It has been a few years since I was into a cluster. It would be nice if someone with a cluster for a 3.07 rear could count the teeth on their gear for confirmation, but it sounds like you may have found your problem.
Its nice that you have a brass gear. On my 126 that gear is made of soft plastic and is missing 3 teeth. I have worked on other vdo speedometers where the drive gear was plastic, they never seem to last.

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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