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Thread: Rear End Gearing

  1. #1
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    Default Rear End Gearing

    Does anyone know what gearing came with the Cabriolet stock? I haven't had a chance to get under mine recently and I'm trying to calculate gearing for a Cummins swap

  2. #2
    Senior Member Sucked In
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    Default Re: Rear End Gearing

    there is no easy way to know and it depends on what year your truck is. check the doorjam for the axle code and you are likely to be able to find the information that you're looking for.
    There are no rules in life. You have to think!

  3. #3
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    Default Re: Rear End Gearing

    Will do. Thanks for the reply

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    Default Re: Rear End Gearing

    I agree there is no set gear ratio they used.
    Jack up one wheel.
    Spin it a full revolution.
    Count exactly how many times the drive shaft turns.

    You will end up with 4.1 turns or 3.75, meaning 4.10 ratio or 3.73 ratio.
    If it is a limited slip rear end I think it is doubled? So you will count 8.2 turns of the drive shaft for a 4.10 ratio.

  5. #5
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    Default Re: Rear End Gearing

    hi, im interested to know what problems you think the diff ratio might course you after fitting a cummins into your cabriolet as ive just fitted one into my cabriolet complete with an allison 4 speed AT545 box, i havn't road tested it yet as ive got a problem with the power steering box. as soon as iv'e replaced it i'll try it out. have you actually started the engine swap?

  6. #6
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    Default Re: Rear End Gearing

    No I haven't. I'm planning on cutting out some of the flooring and doghouse to fit a NV5600 in it. The issue I was concerned with was the IDI's (And presumably) 460's can run more RPM's than the 6BT's can. I didn't want to have the motor completely strung out at 65MPH where as the IDI I have now, along with a C6 is getting close at about 70MPH.

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    Default Re: Rear End Gearing

    right good point, not sure on that how mine will pan out until i road test it. if i find its an issue with the top end i may have to consider some work with the final drive, but for now i just need to get this truck on the road to give it a shake down incase their are any issues with the engine and box or indeed any of the fabrication work i've done. i hope not because im desperate to get the truck back on the road were it belongs instead of in my driveway in bits. let me know how you get on with the swap.

  8. #8
    1995 5speed PSD Sucked In bob_95_psd's Avatar
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    Default Re: Rear End Gearing

    I wrote a little Excel macro that I use to determine axle ratio or RPMS. It's pretty basic math but it works.

    Basically there are five factors involved.
    • Speed
    • RPM
    • Tranny ratio (any gear)
    • Axle ratio
    • Tire Diameter

    Knowing any four of the five my macro will calculate the missing factor. With GPS accurate speed is easy to get and most OD trannies are 1:1 in third gear and if not you can easily get the final ratio off the internet for pretty much any transmission.

    Like I said I did it in excel but if anybody wants it just shoot me a message with your email address and I'll send it along.

    Bob
    Last edited by bob_95_psd; 06-08-2012 at 08:56 AM.

  9. #9
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    Default Re: Rear End Gearing

    Theres also:

    http://www.grimmjeeper.com/gears.html

    I'm not sure what tire setup you have on yours but going off of a 40" tire and a 4:10 rear end you'll be at ~2200 RPM in fourth at 65MPH vs. 1600 RPM with a G56 in sixth gear.

  10. #10
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    Default Re: Rear End Gearing

    If your Cabriolet is the same as mine it should be a 4:10 Dana 70HD

  11. #11
    Junior Member New User Ben's Avatar
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    Default Re: Rear End Gearing

    well you can call ford and ask them , probably 4.10, just like mine

  12. #12
    Junior Member New User Ben's Avatar
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    Default Re: Rear End Gearing

    most are 4.10

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