The first thing I would check is the driven gear at the transmission. My 52 had a driven gear that had plastic teeth. They had worn, chipped, and finally broken. If that isn't it, it is probably the speedometer it self. It wouldn't hurt to pull the speedo, and clean/rebuild it anyway. The grease in my speedo had hardened and was a pain to get out. But once cleaned and relubed it worked like new. If you do it yourself, just go slow. There are some delicate parts that won't take rough handleing. Good luck.
