[oldgold70c10]

OK, do this then:
find something that you can use as a handle like an allen wrench & put it in the back where the speedo cable goes on the speedo. Turn it 100 turns. You've got to turn it forwards, because there is a ratchet-like device in the odometer reel that prevents miles from being subtracted in reverse (remember Farris Beuller's Day Off?) The odometer should move 1/10th of a mile. You could do this 10 times if you've got the patience and every gear in the thing would have gone round at least 1 turn & it would be 1 mile from where you started.
Anyway, if it passes that, get a short piece of old speedo cable or something that will let you spin it with a drill motor without damaging the square hole that the speedo cable goes in. I like using a small flat blade screwdriver bit. Run it at about 60 mph and watch the odometer. It should do one mile in one minute. If the odometer pauses or does less than 1 mile, somethings wrong. If everything checks out OK it your speedo cable or speedo gears in the transmission.