Okay, so I've been trying to logically figure out this question but I'm still uncertain. So if I'm supposed to have a 17 tooth speedo gear but I am running a 21 tooth, will that make the speedometer appear to run slower or faster than my actual speed???
Changing the ratio between the drive gear and the driven gear will change the speedometer reading. A driven gear with fewer teeth will speed up the speedometer reading; more teeth will slow down the reading.
Keep in mind that sometimes you also need to change the drive gear in the transmission to get the calibration correct. Otherwise, drive and driven gears that are not compatible will chew themselves up in short order.