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Speedometer worm gear ratio changes

  1. Jul 22, 2014 #1
    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.
  2. jcsd
  3. Jul 22, 2014 #2
    The center to center distance stays the same. It is the outside diameter of the driven gear that changes. Even so there is a limit to how far one can go without having to change the drive gear. The teeth can still be cut to fit the worm gear.

    I have always presumed that it was the difference in outside diameter of the driven gears that made it all work.

    I remember that the old Muncie four speeds had a about six different numbers of teeth on the driven gear that it could accommodate without having to change the drive gear.

    We can thank the aftermarket for the nine zillion combinations that are available.

    The newer cars have gone electronic with a magnet located on a gear and no cable. Some new VW's even use one of the antilock brake sensors to send the signal.

    Edit: OK those little alarm bells in my head just rang. The center to center distance stays the same because the hole in the side of the transmission is eccentric to the drive gear. Turning the driven gear housing to the right makes the gears mesh. This allows several size gears and numbers of teeth to be used.:redface:
    Last edited: Jul 22, 2014
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