What I would do... Remove the speedometer and take it apart and see what you can find. It may only need cleaning. Gears in it turn the odometer and may be gunked up a bit. Here's a link to some photos of my 56 speedometer (I expect 55 would be the same).
https://1drv.ms/f/c/dd6d72779f5026a2...O-dwQ?e=Xg9dx9