Thanks for the comments. I've seen notes here on that problem: you replace the gear, it doesn't work for a few/20/100/more miles; then it starts working. I had that happen once but it started working in less than a mile.

I had another one d it; work for ~500 miles, then quit. Removed it to find the gear was bad.

More often, I've had this: I replace a gear and the speedometer works for 9/10 mile every now and then.

A couple of these recent units I put in cars and drove to no avail.

What was consistent here, multiple times, one odometer: Gear A, odometer doesn't work; Gear B, odometer works; Gear A, odometer doesn't work; Gear B, odometer works; ... repeat a few times.

I can understand how a slightly-oversize new gear might bind; when I don't get is why a new gear would work but a used one wouldn't - when that same used one worked in a different odometer.
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240 drivers / parts cars - JH, Ohio