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    Author Topic: Determining axle ratio  (Read 862 times)

    Offline Purple73Cuda

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    Determining axle ratio
    « on: September 23, 2009 - 07:47:22 AM »
    What is the most accurate way of determining axle ratio short of pulling the pumpkin out of a 8 3/4 rear? I marked the drive tire and driveshaft and turned the driveshaft and counted the turns for one tire revolution. I got 3 1/4 turns. I said great it is a 3.23. I changed the trans. and before I put the new trans. in I bought a 33 speedo pinion in. I drove the car this weekend and compared the speedo to the GPS. The speedo was reading 12 MPH low. So I went and pulled the speedo gear out of the old trans. and I found it had a 28 tooth gear. With the 28 tooth gear it read within a mph or 2. I looked in the chart and the 28 tooth is for a 2.76 rear.
    So, is the method I used wrong? Does the method work on a non Suregrip rear? My car is a one legger. It obviously didn't work for me. Am I missing something?
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    John
    '73 Cuda 340
    '70 RR 440+6
    '66 Charger 383


    Offline Bullitt-

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    Re: Determining axle ratio
    « Reply #1 on: September 23, 2009 - 09:57:19 AM »
    Tire diameter has to go into the calculation.... Do you have a Tach? 

    Enter #s in one of the charts linked to on this page http://www.moparts.org/Tech/tech/pages/speedo.html
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    Wade  73 Rallye 340..'77 Millennium Falcon...13 R/T Classic   Huntsville, AL
    Screwed by Photobucket!

    Offline Purple73Cuda

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    Re: Determining axle ratio
    « Reply #2 on: September 23, 2009 - 10:25:08 AM »
    I have a factory tach. I have F70x14 Polyglas tires. What is the metric equivalent?
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    John
    '73 Cuda 340
    '70 RR 440+6
    '66 Charger 383

    Offline Bullitt-

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    Re: Determining axle ratio
    « Reply #3 on: September 23, 2009 - 10:39:54 AM »
    215/70-14   approx. 26" tall
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    Wade  73 Rallye 340..'77 Millennium Falcon...13 R/T Classic   Huntsville, AL
    Screwed by Photobucket!

    Offline Chryco Psycho

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    • 70 Challenger R/T SE 70 tube Chassis Cuda now sold
    Re: Determining axle ratio
    « Reply #4 on: September 23, 2009 - 10:58:50 AM »
    the trick with the rear diff is to have both tires turn exactly the same amount or if it is aan open diff / no sure grip , block one tire & count the # of turns of the driveshaft to make the one tire go around twice
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    Challenger - You`ll wish You Hadn`t

    Offline Purple73Cuda

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    Re: Determining axle ratio
    « Reply #5 on: September 23, 2009 - 12:52:41 PM »
    CP,
    Maybe that is what I did wrong. I did not block the non drive tire.
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    John
    '73 Cuda 340
    '70 RR 440+6
    '66 Charger 383

    Offline shadango

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    Re: Determining axle ratio
    « Reply #6 on: September 23, 2009 - 05:41:05 PM »
    Quote from: Chryco Psycho on September 23, 2009 - 10:58:50 AM
    the trick with the rear diff is to have both tires turn exactly the same amount or if it is aan open diff / no sure grip , block one tire & count the # of turns of the driveshaft to make the one tire go around twice

    When I was messing with my open rear (sounds gross) I found I had to turn both wheels t the same time to get the right ratio.....my diff has a tag hanging on it and I just wanted to confirm it.....if I turned only one wheel I got some really oddball number. Then tried turning both at the same time (both marked) and I got 2.75 turns..... :2thumbs:
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