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Technical Ford Speedometer Drive Question

Discussion in 'Traditional Hot Rods' started by FlatJan, Mar 4, 2018.

  1. FlatJan
    Joined: Dec 13, 2013
    Posts: 278

    FlatJan

    Hi Gang,

    I want to run a closed drive line banjo on an AV8 with a Ford speedo.
    how do i know which speedo drive to choose?
    There are 18, 19 and 21 teeth as far as i have seen.
    i guess it depends on the differential ratio and tire size?!
    Which combinations needs which turtle?

    thanks guys´n gals
     
  2. alchemy
    Joined: Sep 27, 2002
    Posts: 17,270

    alchemy
    ALLIANCE MEMBER

    They aren't expensive, so grab one and give it a try. Then you will know which way you need to adjust, if possible. You might find that none are gonna get it perfect (I did). You might need an inline ratio change box. A good speedo shop can make a box that will give you a perfect reading speedo.

    The gear on the driveshaft is the same for all the turtles.
     
  3. FlatJan
    Joined: Dec 13, 2013
    Posts: 278

    FlatJan

    ok will do
    thanks alchemy
     
  4. The Green Bible has a table showing which one you need for stock ratios and tires. If you post your rear end ratio and tire size maybe someone could help. The turtle should match the teeth on the gear.

    Charlie Stephens
     

  5. topher5150
    Joined: Feb 10, 2017
    Posts: 2,519

    topher5150
    Member

    I have a turtle assembly, and the worm gear if you still need one. I'll have to double check the number stamped on it.
     
  6. alchemy
    Joined: Sep 27, 2002
    Posts: 17,270

    alchemy
    ALLIANCE MEMBER

    Yes, you don't pull the gear out of the turtle. You just switch out the whole turtle. Each turtle housing has a gear centerline a bit different from the other to make the driven gear spaced properly from the driveshaft.
     
  7. FlatJan
    Joined: Dec 13, 2013
    Posts: 278

    FlatJan

    @Charlie Stephens: i found the speedometer cable table but not the one for turtles. i can see it referenced as " speedometer gear chart" but where is it hidden?
    thanks
     
    Last edited: Mar 18, 2018
  8. Look on page 632 of the Green Bible.

    Charlie Stephens
     
    FlatJan likes this.
  9. FlatJan
    Joined: Dec 13, 2013
    Posts: 278

    FlatJan

    thanks charlie i have an old original Ford chassis catalogue. it doesn´t have that page..
     
  10. Bruce Lancaster
    Joined: Oct 9, 2001
    Posts: 21,682

    Bruce Lancaster
    Member Emeritus

    I'm at work with no catalog...but try the 17,000 general part number section, where speedometers live along with other small systems. Unless using actual original sizes, try to match your tire diameter to an original size in the diameter and ratio chart.
     
    FlatJan likes this.
  11. Relic Stew
    Joined: Apr 17, 2005
    Posts: 1,147

    Relic Stew
    Member
    from Wisconsin

    Most speedometers are 1000 rpm = 60 mph. If you know your tire revs per mile (some manufacturers have that info on their website) and axle gear ratio, you can figure out the speedo gears necessary.
     
    FlatJan likes this.
  12. FlatJan
    Joined: Dec 13, 2013
    Posts: 278

    FlatJan

    thanks for the hints guys :)
     
  13. Moriarity
    Joined: Apr 11, 2001
    Posts: 21,895

    Moriarity
    SUPER MODERATOR
    Staff Member

    3.78's take a 19 tooth turtle, 3.54 is 18 and 4.11 is 21. this of coarse is assuming you have stock diameter tires
     
  14. Bruce Lancaster
    Joined: Oct 9, 2001
    Posts: 21,682

    Bruce Lancaster
    Member Emeritus

    If you are using tires that are not on the Ford chart, you will need to find out diameters of the tires shown and match up your tires. This is an inexact science since tires are not all that uniform, especially over several decades, but you should be able to come close by just asking Hambers about sizes of particular tires.
     
  15. hightower611
    Joined: Dec 7, 2009
    Posts: 125

    hightower611
    Member

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