Weird speedometer issues

When I first got my car I checked the speedo with a GPS app and found it to be pretty accurate.

Shortly after replacing the engine and rebuilding the trans, the speedometer needle started jumping all around. I pulled the cable out of the back of the speedometer and squirted it with silicone spray lube.

The jumping needle is now an occasional problem, but the rest of the time the speedometer reads steady, but much faster than actual speed. Last weekend I was reading 100mph on the interstate, when I was only going about 70.

I'm looking to replace the cable, but I can't see how a bad cable could make the speedometer read TOO FAST? Did the bouncing needle kill the speedometer?