Speedometer rests on 100...

Actually, you might only have to wiggle it, or push it a little further in from the firewall side. Read post #5
But if THAT doesn't work then try disconnecting it.
But I would just slap the top of the dash; if the needle drops, that pretty much proves the bearings are loose,the torsion-spring is broke, or the cage is heavily stressed or broke, or I suppose the speed cup is no longer round,or......... oh heck just take it out and look,lol.