Please Help!! Speedo gears,

Discussion in 'Troubleshooting & Diagnosis' started by Michaels, Apr 22, 2021.

  1. Michaels

    Michaels Member

    Messages:
    51
    Likes Received:
    6
    Joined:
    Aug 11, 2019
    1980 Camaro
    hi guys I recently had a tranny shop rebuild my TH350 and install 3:73 gears in the rear end. he said we got the speedo as close as they could. but if I drive 100 km it reads 60km
    thats way off. the calculators for some reason are not working for me.
    i want to change the drive gear and the driven gear.
    my tires are BFG T/A Radial 255/65R15
    my TH350 has the larger gear with the 2" housing

    thanks guys 815eW05ZGvL._AC_SL1500_.jpg
     
  2. 70-camaro

    70-camaro Veteran Member

    Messages:
    3,271
    Likes Received:
    1,553
    Joined:
    Mar 25, 2010
    Location:
    marietta ohio
  3. Michaels

    Michaels Member

    Messages:
    51
    Likes Received:
    6
    Joined:
    Aug 11, 2019
    thanks but that does not work??
     
  4. Gary S

    Gary S Administrator Lifetime Gold Member

    Messages:
    23,812
    Likes Received:
    1,459
    Joined:
    Apr 14, 1999
    Location:
    Bismarck, North Dakota
    I don't use those calculators. I do it by simple math. If you drive 100kph and the speedometer reads 60kph, it is slow by a ratio of 1.67 so it needs to have the gear ratio changed by that amount..
    Count the teeth on your existing drive and driven gears and find new gears that increase that ratio by 1.67 and it should be accurate.
     
  5. DougD

    DougD Veteran Member

    Messages:
    105
    Likes Received:
    29
    Joined:
    Dec 2, 2014
    Location:
    Bradenton, FL
    FWIW I found that I needed to swap both gears to get the magic combo. Play around with the counts in combination via any of the calculators.
     
  6. Twisted_Metal

    Twisted_Metal Administrator Staff Member Lifetime Gold Member

    Messages:
    29,291
    Likes Received:
    2,257
    Joined:
    Feb 26, 2004
    Location:
    Bloomington, MN
    If you have the proper speedometer gears... It could be a speedometer issue.

    Normally they bounce or quit working entirely as I've never seen one that was consistently off unless the wrong gears were installed.
     
  7. Michaels

    Michaels Member

    Messages:
    51
    Likes Received:
    6
    Joined:
    Aug 11, 2019
    thank you guys, I found a you tube calculation that says I need a 15 tooth drive gear and a 43 driven gear. waiting on parts, once installed I will report.
     
  8. 1320feet

    1320feet Veteran Member

    Messages:
    357
    Likes Received:
    146
    Joined:
    Oct 23, 2019
    Location:
    Calgary, Alberta.
    Sounds like they set it up for MPH.
     
  9. Gary S

    Gary S Administrator Lifetime Gold Member

    Messages:
    23,812
    Likes Received:
    1,459
    Joined:
    Apr 14, 1999
    Location:
    Bismarck, North Dakota
    MPH or KPH shouldn't make a difference for Chevy speedometers. They use the same gears and just paint different numbers on the face of the dial.
     
  10. rocket dawg

    rocket dawg Veteran Member

    Messages:
    426
    Likes Received:
    50
    Joined:
    May 5, 2015
    Location:
    Grand Rapids Mi
    A GPS helps while getting it dialed in. I changed gears in a TH350 years ago. It's dead nuts on with the GPS at 60 mph.
     

Share This Page