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Thread: Calculated tire size question.

  1. #1
    Potential Tuner 70zman's Avatar
    Join Date
    Dec 2013
    Location
    Cranbrook BC
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    4

    Calculated tire size question.

    When I put in my new tire size, 28.9 inch with 4.10 gears (original ratio), the calculated tire size goes to 29.32 inch. My brother has HPTuners also and when he put in a tire size of 33.5 with 4.10 gears, it calculated it at 31.8, when I input it on my program it calculates 33.99 inch. Why does the tire size recalculate? And why the difference between our programs?
    70 Chevy C20 lowered, 99 Silverado 5.3L, shaved/ported 243 heads, Stage 2 cam,Custom tune, dual exhaust with H, 4L60E 3000 stall, Corvette servo

  2. #2
    Advanced Tuner
    Join Date
    Apr 2015
    Posts
    302
    I would have thought it had to do with a final drive ratio number, but that doesn't explain the difference between the two programs running the same numbers. Unless the two programs are using two different transmissions.

    More to the point the number calculation may take in the tire size, gear ratio, and the transmission ratio somewhere.

    I'm just guessing though, someone correct me if I am wrong

  3. #3
    Tuner
    Join Date
    Mar 2015
    Location
    florida
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    110
    mine does it too might have something to do with a mean of tire size averages that's my guess

  4. #4
    Senior Tuner Russ K's Avatar
    Join Date
    Dec 2005
    Location
    Regina, Sask.
    Posts
    4,213
    To correct the calculated tire dia; calculated tire dia/current tire dia*trans revolutions/mile. Then close & open the speedo page. Now the calculated tire dia should match the current tire dia. This is very important, as the trans shift speeds depend on the correct calculated tire dia & trans revs/mile.

    Russ Kemp