blkgt50

I know that you guys are going to tell me to look at the speedometer recalibration gear section but I have a little problem. I bought an 88 mustang with 5 spd. The guy that had it before me had 3.55 gears in it. the problem is that I think that the people that did for him put in the wrong gear (23 tooth). I know have 3.73 gears and want to know what'd to make sure that I'm going to get the right gear. According to the topic it says that I need a 21 tooth gear. Is that with 15 inch rims or is that with 16's? Also dose the speed of the gear dectate the how the tach reads?