the gear size determines how fast the speed spins. any gear should work, but only one wil be accurate for any particular tire size. there's a chart somewhere showing which one goes with what tire.
the important thing is that any gear should give you a reading of some kind (assuming everything else is working properly). the speedo housing you installed has markings on it and you need to rotate it to the appropriate side. The rotation brings the gear closer or further away to the main shaft inside and affects tooth engagement. if you clean up the metal speedo housing, you should see little numbers on it.