When changing rear end gear ratios, or tire sizes on a vehicle, you can buy different speedometer gears with tooth counts to restore speedometer accuracy. The worm(driver) is inside the transmission and is not changed. It is a 7 or 8 tooth in my application. The speedometer gear(driven) is replaceable and comes in 15 to 23 tooth counts. Just wondering which engineering design principles are allowing tooth changes in the driven gear without any changes in the driving worm gear or in the center to center distances of the gear set. Some design parameter is taking a beating i suspect.