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Thread: LBZ tire size change

  1. #1
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    Default LBZ tire size change

    I know this should be a simple change, but I need to make adjustments for larger tires on an 06 LBZ. Looking over the available options in the tune file, the only thing I can see at this point is table H0136 (Tire revolutions per Kilometer) under the speedometer section.

    Do I just need to calculate the % difference between stock tire size and current size, then adjust the H0136 accordingly? If so, will this correct speedometer, odometer, etc...?
    Richard -
    2008 Trailblazer SS - TVS1900

  2. #2
    Forum tyhee Site Admin GMPX's Avatar
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    Yes, the fancy calculator is not implemented in the LBZ because it is missing a few parameters that we need.
    I don't know the exact formula, but someone over at Diesel Place has done the % change and it seemed to work O.K

    Cheers,
    Ross
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    For tuning support please post your questions on this forum (or other auto forums).
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  3. #3
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    Great. Thank you Ross. I figured it was something like that since the LBZ software is still undergoing changes. I just wanted to double check.

    Have a great day and thanks for an excellent product.

    Richard
    Richard -
    2008 Trailblazer SS - TVS1900

  4. #4
    Junior Member Turbo595t's Avatar
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    Hello,

    I did a tire size change on a LBZ last week. The rev/km that gm has was 424, I calculated the stock tires to be 411 rev/km and the new 285 size tire to be 382, but when I put the 382 in the speedo was high reading 69mph when it was supposed to be 65mph, so I added the 13 rev/km from what I calculated and used 395 rev/km and it was fine.

    Did you run into something like this?

    Thanks,
    Brian
    I would rather be Blown, then bottle fed

  5. #5
    EFILive Distributor Trippin's Avatar
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    395 is what I had calculated/used for 285 75 16.
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  6. #6
    Junior Member Turbo595t's Avatar
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    Quote Originally Posted by Trippin
    395 is what I had calculated/used for 285 75 16.
    I was in a hurry so I googled some tire calculators and first one I used came up with 382rev/km and so did the second. So I broke down and did it by hand and a calculator and I came up with 381.72rev/km........ so I don't know what's going on.

    (285 * .75)+2 = total side wall height

    16 * 25.4 = wheel dia mm

    add those together then divide by 2 to get the radius.

    Then 2pie(r) for the circumference

    then divide into 1000000 millimeters for revs/km

    I came up with 381.72rev/km.

    I could have used pie(dia) for the circumference, but for some reason I have always used 2pie(r)

    Brian
    I would rather be Blown, then bottle fed

  7. #7
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    Yes, Turbo595t, I had the exact same results and the same calculations. I had already calculated the stock 245.75.16 tires out to 411. When I saw 424, I thought that the speedo must have been high from the factory. I was moving to 305.70.16 tires, which calculated out to 382 revs. Afterwards, I decided to take the ratio of my calculated tire heights and apply it to the mysterious 424 number that was originally in the PCM. I multiplied 424 by .92944 (which is 382/411) to end up with 394, which put it just right.
    Richard -
    2008 Trailblazer SS - TVS1900

  8. #8

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    I know my speedometer read about 68 mph when my GPS said 65 mph, so the factory numbers do indicate more miles traveled than actual, resulting in a better "perceived" fuel mileage.

  9. #9
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    I figured out on the Revs per mile that going from a 245 to a 285 was about -8%

    So I just -8% the rev table. GPS put it pretty much right on

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