Make sure the cable didn't pop off while putting it back together. I have had a time with them either not staying on or sometimes I can't get them off the speedo itself. Check the speedo gear that goes into the tranny. Make sure it isn't stripped. Then I would assume its the speedo.
put the car on jack stands, take out the speedometer and then start the car. Put it in gear and let the wheels turns. Then u can see if the speedo cable is actually spinning inside the sheath. This way you can see if it is related to the drive gear, driven gear or the cable and not the actual speedometer.
then go from there. I have found that if the needle is not pressed on the speedo shaft hard enough, it will sit there and ride on the shaft that turns it and makes it read.
or, i have taken the end out of the trans. chucked it in a drill and tried to spin it that way. but that wont tell you anything about the drive gear in the trans.
any more questions, let me know. I have spent alot of time here recently messing with my speedo, gear and cable.
Take the cable end out of the trans. and look to see if the teeth are stripped off the plastic gear. If they look fine, stick your finger in the trans. and see if the teeth on the drive gear feel fine (put it in neutral and rotate the driveshaft so you can feel the complete gear. If this all looks good, take the little plastic gear off the end of the cable (held on by a little c clip the you just pull off) and look inside it and see if you still see a square socket in the plastic for the metal cable to go into. This could easily round out and the gear would just spin around the cable and not actually engage and spin the cable itself.