Speedometer Drive Gear

Discussion in 'Troubleshooting & Diagnosis' started by tnt30143, Aug 21, 2017.

  1. tnt30143

    tnt30143 New Member

    8
    0
    Jul 15, 2015
    Jasper, Georgia
    Hello all. I tried searching but couldn't find an answer. The speedo quit working in the 1980 Z. I did the cable spin and the gauge worked, so I went to the next easiest and changed the driven gear. Got the correct color and number of teeth, put it in and it still doesn't work. Next step is drive gear. Is there any way to tell the color/number of teeth without actually pulling the tailpiece? I don't really want to pull it more than once, and my son doesn't want to not be able to drive it while it's waiting for the part. I tried the formula on the TCI site but I'm not sure my algebra came out right. It has a 22 teeth driven gear, the formula comes up between 8 and 9 teeth on the drive gear, a little closer to the 9 side. Then on one of the threads I found searching said something about a 3:1 ratio which would make the drive gear 7 teeth. Any help is appreciated.
     
  2. RS1979

    RS1979 Veteran Member Lifetime Gold Member

    3,175
    447
    Jun 18, 2013
    Memphis, TN
    Does your car have cruise control? My '79 came with it and I had the same problem your having. I removed both of the original cables and replaced them with the non-cruise control cable. Works fine now.
     
  3. tnt30143

    tnt30143 New Member

    8
    0
    Jul 15, 2015
    Jasper, Georgia
    No, no cruise control. Only "luxury" option this thing has is delay wipers.
     
  4. RS1979

    RS1979 Veteran Member Lifetime Gold Member

    3,175
    447
    Jun 18, 2013
    Memphis, TN
    In that case, you can remove the driven gear and the bullet. Then look inside the hole and see what color it is.
     
    Blown Camaro likes this.
  5. tnt30143

    tnt30143 New Member

    8
    0
    Jul 15, 2015
    Jasper, Georgia
    I wasn't sure if I could actually tell anything looking through that hole. I'll try that and see if I can see anything. Thanks.
     
  6. Lou6t4gto

    Lou6t4gto Veteran Member

    325
    15
    Aug 18, 2010
    Ocala, Fla
    might just be the drive gear slipped off, it only has a little spring clip holding it on.
     
    Blown Camaro likes this.
  7. MidniteZ

    MidniteZ Veteran Member Lifetime Gold Member

    610
    9
    Oct 4, 2005
    Cedar Rapids, Iowa USA
    This happened to my wifes 79. I ended up just getting a new gear. This is the site I use to figure out my gearing.
    http://novaresource.org/speedo.htm
     
    Coadster32 likes this.
  8. Coadster32

    Coadster32 Veteran Member Lifetime Gold Member

    1,487
    87
    Oct 2, 2013
    Milford,CT
    Thanks for the speedo calc link. Handy indeed.
     
  9. Cardinal

    Cardinal Veteran Member Lifetime Gold Member

    8,249
    15
    Jun 22, 2003
    Endicott, NY
    Here's a the link to a post I made YEARS ago. It may sound difficult but when you get used to it, it really isn't that difficult.

    http://nastyz28.com/threads/can-someone-make-a-sticky-on-speedometer-gear-calculations.150376/

    Here's how to calibrate a speedometer:

    1. You HAVE TO know the rear gear ratio and the tire size. From that you get a final drive ratio using a formula that resides at bowtie overdrives http://www.700r4.com/speedoCalc/speedocalc.shtml

    Once you know the speedometer drive ratio, you then look on my chart below for the drive and driven gears that are closest to that number. EX: you plug in 2.35/70/15 tires with a 3.73 rear gear. The formula tells you that you need a 19 drive and a 34 driven gear which don't exist. 34 divided by 19 = 1.789 Look on the chart and a 15 drive and a 25 driven gear set will work.

    [​IMG]
     
  10. Cardinal

    Cardinal Veteran Member Lifetime Gold Member

    8,249
    15
    Jun 22, 2003
    Endicott, NY
    Here's a the link to a post I made YEARS ago. It may sound difficult but when you get used to it, it really isn't that difficult.

    http://nastyz28.com/threads/can-someone-make-a-sticky-on-speedometer-gear-calculations.150376/

    Here's how to calibrate a speedometer:

    1. You HAVE TO know the rear gear ratio and the tire size. From that you get a final drive ratio using a formula that resides at bowtie overdrives http://www.700r4.com/speedoCalc/speedocalc.shtml

    Once you know the speedometer drive ratio, you then look on my chart below for the drive and driven gears that are closest to that number. EX: you plug in 2.35/70/15 tires with a 3.73 rear gear. The formula tells you that you need a 19 drive and a 34 driven gear which don't exist. 34 divided by 19 = 1.789 Look on the chart and a 14 drive and a 25 driven gear set will work.

    [​IMG]
     

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