TRANS CHANGE AND REAR GEARS, speedometer question

Discussion in 'Transmission & Driveline Topics' started by Lou6t4gto, Jan 24, 2018.

  1. Lou6t4gto

    Lou6t4gto Veteran Member

    183
    5
    Aug 18, 2010
    Ocala, Fla
    I just changed from a th350 and 2.73 gears to a 700R and 3.42 gears. the speedometer gears are still set up for the 2.73s. (I'm waiting for a new driven gear "Housing". About How Much MPH speedometer "error" is there now ?? Until I get the gears changed Thanks
     
  2. camaro71/holland

    camaro71/holland Veteran Member

    903
    24
    Aug 27, 2003
    The Netherlands
    Use your phone / get an app that shows you the gps speed and go for a spin.
     
    Dave Nelson likes this.
  3. Dave Nelson

    Dave Nelson Veteran Member Gold Member

    3,261
    500
    Dec 19, 2003
    Vineland N.J USA
    I've used the app several times, highly accurate.
     
  4. Lou6t4gto

    Lou6t4gto Veteran Member

    183
    5
    Aug 18, 2010
    Ocala, Fla
    old fart, no cell phone at all !
     
  5. 70-camaro

    70-camaro Veteran Member

    1,065
    108
    Mar 25, 2010
    marietta ohio
  6. G72Zed

    G72Zed Veteran Member

    409
    28
    Sep 8, 2015
    Canada
    Or "pre" internet days, math formula with the old pen and paper, lol, still works

    Assuming a 27 inch tall tire and not factoring in converter slip %, and a constant rpm of 2,300 for both setup's = + 10 mph

    TH350/2.73 = 68 mph
    700R4/3.42 = 78 mph (OD effective ratio of 2.38)
     
  7. berg2695

    berg2695 Veteran Member

    146
    12
    Nov 13, 2011
    Auburn, WA
    The only thing that matters in this case is the change in the rear gear ratio. 3.42 is 25% more than 2.73. Therefore, the speedo will read 25% higher than the actual road speed. For example: an indicated 50 is actually 40; an indicated 75 is actually 60 mph.
     

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