Hi Novacane,

I would start with the simple questions first like,

What is the size of your rear tires and wheels?
* I've noticed the biggest change in speedometer accuracy has usually been do to the size of the rear tires and wheels.
Is the tranny the original one to the car?
If it is, has it been rebuilt?

This has obviously been an issue in your car since the speedo cable was previously disconnected, but it may be something simple.

Good luck,

Greg