I had purchased a speedometer ( 125 MPH ) a while back. MPH worked, but not the mileage. Some of the gears were like over dunked cookies. I finally got everything back together with new gears. I spun the small pinion type gear, the odometer advanced fine. But, installed in car it does not work. What makes that pinion shaft spin ? Any ideas ?

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1984 MB 300SD Turbo