Speedometer Issues

Looking for advice before I go tearing my dash apart:
Currently the old speedo cable is connected to the transmission but disconnected from the back of the gauge. The needle is pointed around 10MPH. I can't spin the cable by hand, so it seems to be engaged in the transmission.
If I put a new cable into the back of the gauge and spin the free end by hand, I can make the needle jump, but it always settles back around 10MPH instead of 0.
I tried to reconnect the old cable and take the car around the block and got no needle movement, but also the clip is broken off the old cable, so it's hard to know whether it was fully seated in the gauge.