I don't know why it does this. The first time about a year and a half ago it stopped working it fixed itself within a day. The next time....about six months ago it stopped working for about two and a half months and then magically it started working again. I know this must mean that somewhere something is loose but what? I can't even begin to guess where to look. All I know is that the speed sensor and the speedo gear are fine. Could it possible be the speed calibrator that I had installed along with my 4.11 gears two years ago? I just want it fixed cause getting a ticket for a malfuncitioning speedo sucks. Please help.