Speedo Gear Replacement - Gear Trashed!

[wiski]

I had 3.55 gears installed into my Mustang over two years ago. At some time (not sure if it was before or after) I remember my speedo bouncing a little bit. Sometimes it would even drop to zero, then jump back to normal. I've replaced my old speedo (101K) for a "new" one (59K). Both speedos seem to work correctly (attach a drill and watch the steady speed readings). I replaced my old speedo gear with a new white one a while ago, but the gauge problem never cleared up. The car isn't a daily driver so I've dealt with the speedo issue for a while, but got mad at it earlier and pulled the gear out of the tranny to look at it. I was expecting my speedo drive gear (inside the tranny) to be torn up, but it looks great! The speedo gear (attaches to the speedo cable) is completely trashed! I'll buy a new one, but why would it have gotten so trashed in the first place??????

 

[jrichker]

Count the number of teeth on the white gear. Some white gears had 23 teeth and were meant to be used with 4.10 rear end gears. The 23 tooth white gear seems to self destruct in some instances.

See http://www.fordracingparts.com/download/charts/129.pdf for help to choose the correct gear for your car.

 

[MFE92]

It was trashed before you got around the block for the first time. Usually, they somehow keep working. The permanent fix is to match the correct drive gear to the correct driven gear, but changing the drive gear is a PITA. It can be done with the trans in the car but you still have to take off the tailshaft housing, so it's best left for when you have the trans out of the car for something else.