Speedometer cable weirdness

My 66 dart sedan 270, has the original speedo cable. My speedometer has been fully rebuilt by a pro, and it has been working fine for the past 2 years. However now I notice the speedo doesn't go past 55, no matter if I try to go beyond, it doesnt go any further. I took it out and tested it with my drill and it will go past 55.

So that leads me to think the cable is f'd/ So I notice when I removed the dash cluster, the inner part of the cable pulls out. Is that right? I have lubed that cable in the pst, and it is one long piece. But how is it that it can simply slide out ?? If it was busted, I assume the speedometer wouldnt work at all? RIght? Help? And any idea on where to find the right size cable? Everything I find online points me to the wrong length cable. thanks guys