I recommend having the speedometer unit properly serviced. Getting it out of the dash is quite a chore so if you're going to remove it, have it serviced. Attached are a couple pix of a 54 unit. Either the retainer spring on the front of the unit (first pic) has gotten weak, or more likely, the stop (the thinner brass wire in the second has become detached.
The speedometer works when the spinning cable turns a weak magnet that's attached to the tab you see in the second pic that's against the drum. Actually, there's just a little clearance...enough for the tabs to spin and cause the inner drum to turn at a calibrated rate. Over the years, parts wear and most commonly, the axle that attaches the needle to the drum breaks off. That's what happened to this unit. I figured it was time though...it had almost 300K on it...
