Also, it’s easy to mathematicaly calculate a gear change, so you don’t have to guess which gear to buy.

How far is your speedo off, and what speed did you check it at?

Adding one tooth to a 21 tooth gear, (1 divided by 21) will slow down the speedometer by 4.7%

60 MPH minus 4.7% equals 57 MPH. (0.047x60 equals 2.82 MPH change)