I have an 83 so it may be different... but, someone (me) might have accidentally forgotten to put the little retaining clip on the left front grease cap to hold the speedo cable in place once or twice (but who is counting).
Anyway, sounds like yours is different since you mention the trans. I just know that when I get know speedo or odometer readings at all it is because that little cable is no longer firmly attached and spinning around
