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  1. #1

    Default Larger Tires/Tranny Shift Pattern

    I have a 2001 Suburban 4x4 with larger tires, 285 75R16. It also has a 2 in body lift. The speedo is off about 4mph and I know this can be recalibrated, but does anyone know how important is is to change the transmission shifting pattern? Will recalibrating the speedo correct this? I noticed that it has a hard time downshifting, and I'm worries about tranny damage. I've experienced some engine overheating when climbing slowly up a steep hill because I think the engine is working too hard and not downshifting quickly enough, plus I get pinging. I want to avoid using the Tow/Haul mode all the time. Thanks!

  2. #2

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    Quote Originally Posted by BajaDiver View Post
    I have a 2001 Suburban 4x4 with larger tires, 285 75R16. It also has a 2 in body lift. The speedo is off about 4mph and I know this can be recalibrated, but does anyone know how important is is to change the transmission shifting pattern? Will recalibrating the speedo correct this? I noticed that it has a hard time downshifting, and I'm worries about tranny damage. I've experienced some engine overheating when climbing slowly up a steep hill because I think the engine is working too hard and not downshifting quickly enough, plus I get pinging. I want to avoid using the Tow/Haul mode all the time. Thanks!
    I would look into getting the PCM system tuned to see if that could offer you better shifting and spedo calibration for your new tires. Send an email to the guys at PCMforless.com and see what they have to say for example.

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  3. #3
    Legend unplugged's Avatar
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    Default

    Yes the tire size can affect your shift points dramatically. I just swapped my burb tires from 305's to 265's and it definitely pulls better on grades. You can re-cal the PCM so the speedo reads correctly with the bigger tires, and you can get some adjustment of the shift points. Check out Hypertech's info on how this is done.
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  4. #4

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    Also, some new gears might be in order, as larger tires take longer to make on e full turn.
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  5. #5
    Jr. Apprentice
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    Thanks guys, I was looking for this info. And some others... will continue to look for answer on other later.
    I had my 2002 1500HD 4x4 tranny serviced by the dealer( as was still under warranty) at 25K, shortly thereafter (37k) the tranny took a dump. I thought they may have sabotaged it to create warranty work for themselves. It was replaced with a factory rebuilt....eversince when it warms up it starts shifting hard and from dead stop it acts like it slips then lunges. It gets worse as the temperature out here increases...115 degrees in summer. The guage does not register hot. And when I took it back all they could say was they could not replicate the condition..turds did not drive it long enough to heat up. Also they left some bolts out of exhaust shields, stripped the transfer case aluminum fill plug, and exhaust pipe to muffler leak.
    A friend ( boeing engineer) told me about some fluid to use instead of the dexron ( got to remember where I put that info). Also has bigger tires than the bicyle ones they come with..265/R16. Had larger than that, but when gas went up and they wore out I went down some in size.
    So it looks like I should try the reset speedo with apropriate tire size and the shift pattern reset. And maybe the other fluid next time I service it ( and the filter)
    It only does it when it gets hot out...as long as it is under 80 degrees outside she is fine.
    Last edited by vegaschevy; 01-22-2009 at 01:03 PM.

  6. #6
    Jr. Apprentice
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    Thanks guys, I was looking for this info. And some others... will continue to look for answer on other later.
    I had my 2002 1500HD 4x4 tranny serviced by the dealer( as was still under warranty) at 25K, shortly thereafter (37k) the tranny took a dump. I thought they may have sabotaged it to create warranty work for themselves. It was replaced with a factory rebuilt....eversince when it warms up it starts shifting hard and from dead stop it acts like it slips then lunges. It gets worse as the temperature out here increases...115 degrees in summer. The guage does not register hot.
    A friend ( boeing engineer) told me about some fluid to use instead of the dexron ( got to remember where I put that info). Also has bigger tires than the bicyle ones they come with..265/R16. Had larger than that, but when gas went up and they wore out I went down some in size.
    So it looks like I should try the reset speedo with apropriate tire size and the shift pattern reset. And maybe the other fluid next time I service it ( and the filter)
    It onl does it when it gets hot out...as long as it is under 80 degrees outside she is fine.

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