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Thread: Idiots guide to final drive change?

  1. #1
    Tuner in Training
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    Idiots guide to final drive change?

    What are the complete steps to changing the final drive of a 4t65ehd?

    I went from stock 2.93 gearing, over to 3.29.

    Thanks!

  2. #2
    Advanced Tuner arthansen's Avatar
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    Re: Idiots guide to final drive change?

    You made a change that will result in a lower gear ratio so the equation looks like this.

    Old Ratio / New Ratio = Change Factor

    2.93 / 3.29 = .890578

    Now multiply this times the current value in your PCM:

    "Current" * Change Factor = "New"

    Plug the New value in the "New" field and click "Calculate for new value"

    This should put you very close. If you want to fine tune it using your speed, check out this thread.



    Good Luck.
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  3. #3

    Re: Idiots guide to final drive change?

    just remember in the transaxle there is the drive sprocket ratio as well as the differential and the "gear" as reported by the speedo section is the combination of the two.

    eg. a GTP runs a 2.93 diff x 1.121 sprocket ratio = 3.29

    latest version allows change to sprocket ratio.

    I count sheep in hex...

  4. #4

    Re: Idiots guide to final drive change?

    we actually have a 3.29 diff ratio but the sproket brings it back to 2.93, at least in the Impala and Monte per gm parts??? Am i wrong


  5. #5
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    Re: Idiots guide to final drive change?

    Quote Originally Posted by Quick6
    we actually have a 3.29 diff ratio but the sproket brings it back to 2.93, at least in the Impala and Monte per gm parts??? Am i wrong
    Looks like you are right.
    Opening up a stock 98 GTP file shows 3.29 calculated gear but a sproket ratio of 1.121

    My question is how do you tune for 3.29's on a 98 GTP

  6. #6

    Re: Idiots guide to final drive change?

    Change your 1.121 to 1.000 since now our sproket ration matches our diff ratio. My speedo is now correct,


    BUTTTTT it still sets an incorrect gear ratio code which you have to disable. I have informed Chris at HPT about this.
    Rob


  7. #7
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    Re: Idiots guide to final drive change?

    is that all you have to do?
    leave VSS Pulses/Mile, VSS Pulses/Rev and Trans Revs/mile the same?

  8. #8

    Re: Idiots guide to final drive change?

    Correct, also, don't forget to delete the incorrect gear ratio code.


  9. #9
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    Re: Idiots guide to final drive change?

    Quote Originally Posted by Quick6
    Correct, also, don't forget to delete the incorrect gear ratio code.
    Done!

    Thanks for the help

  10. #10

    Re: Idiots guide to final drive change?

    Your welcome!


  11. #11
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    Re: Idiots guide to final drive change?

    I was told that the correct way is to change the Pulses/rev need to be changed from 22.3 stock, to 25 for 3.29.

    Is this correct?

  12. #12

    Re: Idiots guide to final drive change?

    This is what i did, reset the tire size to my stock 27.1 height, set sproket to 1.00, i have auto set trans tables on. I verified my speedo with a GPS, it is dead on. This tells me it is all correct. I did have to adjust my part throttle shift points as they were a tad to high.
    Rob


  13. #13
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    Re: Idiots guide to final drive change?

    maybe that's why you're still setting the code?
    Pulses/rev is reading the relucter.
    If it reads it wrong, wouldn't it pop up the code?

    According to this post here on 3800Tech
    http://www.3800tech.com/forum/showthread.php?t=877

    The proper way is:

    3.29
    Calc'd Pulses per Revolution -25-
    Drive Sprocket Ratio -1-
    Calc'd Pulses Per Mile -23310-

    Stock 2.93
    Calc'd Pulses per Revolution -22.3-
    Drive Sprocket Ratio -1.12-
    Calc'd Pulses Per Mile -23430-

    What do you think?
    Would you be willing to try it and see if it's all ok?

    My trans is being rebuilt right now so I won't be able to try for a bit.

  14. #14

    Re: Idiots guide to final drive change?

    Mine automatically set that parameter. Besides, if it was wrong, the speedo would be off.

    The thread you quoted is from DHP's set-up NOT HPT..

    HPT sets these parameters automatically for you once you input tire size, gear ratio, and sproket ratio.

    Rob


  15. #15
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    Re: Idiots guide to final drive change?

    Quote Originally Posted by Quick6
    Mine automatically set that parameter. Besides, if it was wrong, the speedo would be off.

    The thread you quoted is from DHP's set-up NOT HPT..

    HPT sets these parameters automatically for you once you input tire size, gear ratio, and sproket ratio.

    Rob
    Thanks!

  16. #16
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    Re: Idiots guide to final drive change?

    I wanted to revive this thread.
    I'm now switching back to 3.69's
    What PCM changes do I need to do to make this all work?

    Thanks

  17. #17
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    Re: Idiots guide to final drive change?

    Quote Originally Posted by Overclock_This
    I wanted to revive this thread.
    I'm now switching back to 3.69's
    What PCM changes do I need to do to make this all work?

    Thanks
    This is where I am, lost actually
    I just changed over to 3:69 gears and the speedo is way off and I believe they did not install the 38 tooth reluctor that I paid for so if i can figure this out I will get my money back because I already have the trans back together and I'm driving the car.

    If I change the gears to 3:29 or 3:69 the speedo is still way off but if I set it to 2:93 then at least the tranny will shift correctly and the TCC will actually stay locked while cruising.

    Any help for 3:69 gears?

    Thanks, I appreciate it.

    Thomas
    Best 60' = 1.716
    Best ET = 12.186
    Best MPH = 112.98


  18. #18
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    I hate to bump this after like 2 years but I am putting 3.69's in my car and still am unclear about what to do. Can someone walk me through it specifically?

  19. #19
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    Quote Originally Posted by Andrew
    I hate to bump this after like 2 years but I am putting 3.69's in my car and still am unclear about what to do. Can someone walk me through it specifically?
    I will second that. I have the SRC installed, the speedo seems correct but made no changes in the pcm. Now, the issue is since this was installed, once the brand new tranny is up to temp, it BANGS (and I mean BANGS)gears on lite throttle. Heavy to WOT shifts is fine, cold (open loop) shifts are fine. I have always thought the src did something, but no one else supposedly has this concern.
    So if someone can reshed some lite on the parameters, it would be appreciated.
    TIA - Jody
    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!!!!

  20. #20
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    How many days before someone chimes in?? Alittle help here please......
    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!!!!

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