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??????