It drives the speedo cable. It would be cheaper to grab a display and proximity sensor to count revolutions of the wheel than to build this gear. It has to be ridiculously tall-toothed to register in the perforated disc- looks like a 14.5 degree pressure angle and no longer a standard gear profile. Pretty easy to just bore a flat disc of delrin to the same OD and bandsaw&file the teeth- it doesn't need to be perfect.