I think the VSS, Vehicle Speed Sensor is on the transmission. This would stop your speedo from working, and it would also disable your cruise control if you have one. If it's an auto tranny it would also disable your vehicles ability to go into overdrive, and you'd be at unusually high RPMs at speeds of over 60. But since it's probably not an auto just disregard.
If thats not it you can order a new gauge cluster, which is what people have had to do after botched gauge face plate installs.