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Thread: Idiots guide to final drive change?

  1. #21
    Tuner
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    Jun 2005
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    Okay, TTT again. Lets get the low down on this soon guys. Everyone has differing opinions and I dont know which works.
    2000 GTP- S2X, diamond forged 9.5:1, balanced, Lucas 42.5#, GenV w/N*, stage 3 heads, custom PCM, ZZP SS IC, TOGS, MPS 3.0, Intensified 3.69's tranny - low 12's!!!!

  2. #22
    Tuner
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    May 2007
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    Upstate NY
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    Why is it nobody can answer this question? I've got my tranny ready to install with 3.69's and nobody has an answer?

  3. #23
    Tuner
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    Jun 2005
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    Hutchinson. KS
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    Bandit, I agree 100%. I've been waiting for some better answers also. I installed a SRC and the speedo is correct, but the shifting is another issue. We should be able to correct gear changes via pcm, but no one cares to tell us how!!!!!!!!!!!
    2000 GTP- S2X, diamond forged 9.5:1, balanced, Lucas 42.5#, GenV w/N*, stage 3 heads, custom PCM, ZZP SS IC, TOGS, MPS 3.0, Intensified 3.69's tranny - low 12's!!!!

  4. #24
    Tuner
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    May 2007
    Location
    Upstate NY
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    This is what scott said to use over at 3800pro

    2.93:1 uses 33 and 37 tooth sprockets, while 3.69:1 uses 37 and 33 tooth sprockets.

    If you have HP Tuners, all you need to do is change the sprocket ratio from 1.121 to it's inverse, which is 0.892.

    Be sure to leave your tire size exactly how it's listed in your binary file, and leave gear ratio as 3.29:1 (which is your final drive ratio).

    This is using a stock VSS reluctor, i'll have to try it out, honestly have no idea. I've herd the VSS corrects the speedo but not the transmission settings.
    98 Buick GSX IS3 Heads/GenVblower/cammed/intercooled and stalled
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    94 S-10 with Rancho 3" lift on 33's
    93 Typhoon
    07 Tahoe

  5. #25
    Tuner in Training
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    Aug 2007
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    Aurora, Colorado
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    I hate to dredge this one up, but I am still confused about how this works.

    Assuming that the GP has stock 225/60/16 tires, the tire diameter is 26.6 and the gear ratio is 2.93 for the GTP and 3.29 for the GT.

    Why do the calculated values show incorrectly for tire size and gear ratio in the stock PCM? Do I dare trust that the new PCM values are set correctly if I change the tire size and/or the gear ratio?

    As an example, when changing tire sizes, how can it possibly calculate do the correct calculations for PPM when the original tire size and back calculated values are wrong?

    I have asked DHP, and Bill said the tire size is stored in the PCM, but if the value is not the actual diameter of the tire, what is going to happen if we actually correct the value when changing ratios? Do we use the wrong size as the new value? If we change the tire size, do we use the real diameter, or do we adjust the size using some ratio of "stock wrong size":"stock correct size"

    Help, this is not making any sense to me. How can any of us trust the accuracy of the values being written when the calculated/actual values of the stock PCM do not match the car's real settings. Sure, the HPT may have some factor to correct things, but that factor should be used in the calculated values as well so we see what is actually in the car. If everything were being correctly calculated, I should be able to enter 2.93 gear ratios and 26.6 for tire diameter and nothing would change, try it yourself, the values change. So entering 3.29 gears and 25.7 diameter can't possibly give the correct results.
    Last edited by shor0814; 09-08-2007 at 12:33 AM.
    2007 Corvette Z51
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  6. #26
    Tuner in Training
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    Feb 2008
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    ttt....yeah....anybody know what the "calculated Tire" box means?

  7. #27
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    Feb 2011
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    Canton, Oh
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    Quote Originally Posted by UMD_Jesse View Post
    ttt....yeah....anybody know what the "calculated Tire" box means?
    the calculated tire box is the size of the tire which you can find out by using the tools and conversions calculator

  8. #28
    Advanced Tuner BigDaddyCool's Avatar
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    May 2009
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    Brisbane, Queensland
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    I had this same issue I went from 3.08 to 3.45 and my spedo was out and it also caused issues with shifts etc.....

    I ended up getting mine right by using a tire calc I found on the net. Got it perfect. Just gotta hunt around.

    Simple solution is to find a calculator on the net, find out what your stock car has (tires wise) and run those settings up in the calculator, if they come out exactly the same as whats set stock in the settins then you know your new tire calculation will be correct.

    PS. I messed around for over a year to try and work it out........ in the end a proper calculator on the net worked first go.

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