Results 1 to 4 of 4

Thread: Gear Ratio and Tire Size Does it Effect Transmission Shifitng?

  1. #1
    Potential Tuner
    Join Date
    Dec 2018
    Posts
    3

    Gear Ratio and Tire Size Does it Effect Transmission Shifitng?

    Hello,
    I'm new to HP Tuners. I did search this , but seems like I can't find a definite answer. I have a 57 Chevy with a 2004 GTO LS1/4L460 . We had the harness modified to eliminate the unnecessary and the PCM was flashed with some mods to the tune. At that time we didn't know what tire size or gear ratio we would be running. The tuner put 3.46 for the ratio and a different tire size than what we are running. I know how to modify this in the tune, but does this actually effect the transmission shift points etc? The Dakota Speedo is only off by about 2 mph. So no biggie there I just want to understand if I should correct these values or if it matters? I appreciate everyone's help.
    Thank You
    Matt

  2. #2
    Tuning Addict 5FDP's Avatar
    Join Date
    May 2012
    Location
    Rogers, MN
    Posts
    13,533
    Yes, it changes the shift points.

    The gear/tire wizard has a auto scale feature that you can click and it will change the shift points and mph settings for you based on the new gear ratio and tire diameter. It makes doing this stuff really simple. You can fine tune the settings from there if you please.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.

  3. #3
    Potential Tuner
    Join Date
    Dec 2018
    Posts
    3
    Thanks for responding. I appreciate you help!

  4. #4
    Tuner
    Join Date
    Mar 2017
    Location
    Santa Rosa
    Posts
    99
    Ya but ... I had to put in a tire size 2" different than my actual size to get the speedo in HPtuners to read right. So, double check. You want the speedo in HPtuners to read right - check it against a gps as your driving. The shift tables in the TCM are all speed based (not rpm), so yes, it makes a big difference. But, at 2mph, you'r pretty darn close.
    Peter Young

    1966 LandCruiser FJ40 with 6.0l Gen 3 vortec and 4L80E.
    1984 LandCruiser FJ60 with 5.3l Gen 3 vortec, Magnusen supercharger, and 4L60E