Speedo "stuck"

It's a very close tolerance magnetic coupling.
A pair of little bent metal fingers driven by the cable spins around the perimeter of a metal cupped "disk" which is attached to the needle. If the bearing (nylon?) wears out the tolerance closes and the fingers contact the metal disk. Also dirt can easily be between them giving the infamous vibrating needle. It if touches you have a locked up unit independent of the cable.
(My needle was wobbling today when I first started driving between 60 and 70. Cold parts our first cold day. It got better when it warmed up.)
And when it does lock up under speed you get a trashed needle.
On my square later model ones, I bench test them with a piece of speedo cable end in a drill motor.
Only thing to do with yours is bite the bullet and take it apart and look.
I use LPS1 greaseless to lubricate them. Doesn't attract and hold dirt.