If the speedometer registers speed with the engine running while the car is sitting still, you will first need to make sure that you have successfully completed the calibration procedure. If not, do so and that may solve the problem.
If calibration has been completed, check or run a new ground to the speedometer.
A faulty diode in the alternator can actually cause this symptom. If you rev the engine and the speedometer responds, then disconnect all wiring from the alternator. If the problem goes away, then replace your alternator or have it rebuilt. With only one diode bad in the alternator, you would not necessarily notice a charging system problem yet other than the crazy acting speedometer.
Routing of the signal wire past a high amperage power source or an ignition component may also cause this symptom. Re-route your wiring away from any of these sources.