I'm having problems with my laurel's C31 speedometer, it is moving in a crazy way, is there a way that I can fix this thing?[/quote]\
It would help if you told us what continent, country, state or province, city you were in!
It sounds to me like you have a speedometer shaft to copper drag cup braze or solder joint crack. The shaft driven magnet is spinning. The copper drag cup is wobbling on the shaft and giving erratic position indications to the needle. Go to a speedometer shop and have the copper cup to drive shaft joint redone. Be sure they check the calibration! A small error in location of the copper cup along the drive shaft can make for a major error in speed reading due to incorrect magnetic drag coupling.
If you were in the LA [California for the uninitiated] area, Bob's Speedometer shop on PCH in Redondo Beach does this repair very well. Wherever you are, good luck finding a shop who will talk to you!
EDIT- Before you go to the shop try the simple fix. Could be caused by lubricant in speedometer cable drying out, or a broken cable outer shell. First squirt as much speedometer cable lube as you can into the speedometer end of the cable after disconnecting it from the rear of your speedometer. Work the inner caable up and down to help spread the lube. May cure your problem. If the lube spills out on the floor or onto your carpet, you need to replace the cable assembly. Good luck wherever you are! If no improvement after a few days [to let the lube's graphite migrate all through the cable], go to the speedometer shop.