seedometer quites

I have a 85 supra with a manual transmision. i replaced the speedometer gear on the cable. the gear in the trani is metal. when i get up to 55 in 3rd and push in the clutch the speedometer stops. i pulled the gear out and found that the metal gear had moved back. i put it back in place and it did the same thing. what can i do to keep the gear from moving?