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Thread: Trying to get correct VSS reading on scan tool

  1. #1
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    Default Trying to get correct VSS reading on scan tool

    So I have all my shifts close but all changes were made without changing tire size and pulses per revolution so my vss shows only 16mph at an actual speed of about 65. Is there a way to correct this so I can get the correct speed to show up on my datalog without having to change all my shift points again? I tried using the calculator but that changed it too drastically. My current revs are at 40 and when I change to 17 (or even 16) the speed shows close to actual speed but the transmission doesn't shift. Any ideas?

  2. #2
    Lifetime Member EagleMark's Avatar
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    You could segment swap the stock transmission back in. Then use the speedo calculator for tire and gear size, it will make the changes to all trans shift points as well. I've found if you can get tire manufacturer revolutions per mile it is way more accurate then the tire size, let EFI Live say whatever on tire height as long as RPM and gear ratio are correct.

    Whys it so far off to begin with?

  3. #3
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    I think the guy helping me was playing around and made some changes but he is not around for me to ask. Also I am very much a rookie at this and am teaching myself. So is there not a way to use the speedo calculator but leave all the shifts the same? How would I do the segment swap? Would that basically change it back to stock then I would need to go back in and change all the shifts again?

  4. #4
    Lifetime Member THEFERMANATOR's Avatar
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    Theres no segment swap in the ALLISON TCM. The TCM will not read out the correct speed as it is calibrated for 4.10 gears and 245/75/16 tires fro mthe factory. It does NOT need changed when tire size changes are made like in a gasser. IF you make ANY changes to the speedo portion for revs per mile, ALL the shift points must be MANUALLY changed.
    1995 GMC 2500 SUBURBAN powered by 01 DURAMAX/ALLISON, little of this, a little of that,
    DIAMONDEYE 4" exhaust, CORSA muffler, AFE stage 1 dry filter, custom tuning by me, KENNEDY single pump and pump rub kit.

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    Well that makes sense. So it looks like I will pretty much be starting fresh if I want to be able to watch that on the datalog. How many vehicle speed sensor pulses per revolution is normal for the allison 1000? Also is there a way to figure out he percentage difference from where it is set now to what it should be?

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    Oh one other question. What does the vehicle speed ratio do? It doesn't tell what it does in the description.

  7. #7
    Lifetime Member THEFERMANATOR's Avatar
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    What is this in, and what are you trying to accomplish?
    1995 GMC 2500 SUBURBAN powered by 01 DURAMAX/ALLISON, little of this, a little of that,
    DIAMONDEYE 4" exhaust, CORSA muffler, AFE stage 1 dry filter, custom tuning by me, KENNEDY single pump and pump rub kit.

  8. #8
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    Quote Originally Posted by 406nick View Post
    Well that makes sense. So it looks like I will pretty much be starting fresh if I want to be able to watch that on the datalog. How many vehicle speed sensor pulses per revolution is normal for the allison 1000? Also is there a way to figure out he percentage difference from where it is set now to what it should be?
    Quote Originally Posted by 406nick View Post
    Oh one other question. What does the vehicle speed ratio do? It doesn't tell what it does in the description.
    Good questions Nick. The ppr for the Allison output shaft should be 40. I do not know but am also very interested in what the {H0137} Vehicle Speed Ratio is and how it is incorporated into the calculations.
    2006 Ford F250 Super Duty, 2008 Cummins 6.7, Allison 1000 5spd
    2011 Ford F250 Super Duty, 6.7L Scorpion

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