What's the cure for a wild speedometer?
RAY: The speedometer gauge is attached to the transmission via a cable which--in most cars-- spins inside a plastic sheathing. Your car was built before plastic was widely used, so your sheathing is probably made of stone or something like BX cable. In any case, when it gets old, the cable begins to fray...and when it gets very frayed, the spinning cable gets hung up on the inside of the sheathing. This causes the speedometer to read less than the actual speed. Eventually, the energy from the driven end of the cable forces it to become un-stuck, sending the needle flying up into the Indy range. You need a new speedometer cable. It should be easy to get. I'm sure every 7-11 carries speedometer cables for these '56 Roadmasters.
TOM: I suggest you leave it alone. This old cow probably can't go more than 30 miles per hour any more. At least this way, you can watch the speedometer surge to 60 and pretend it's 1957 again.