th350 speedo gears

Discussion in 'Transmission & Driveline Topics' started by JasonZE, Jul 9, 2019.

  1. JasonZE

    JasonZE Veteran Member

    Messages:
    516
    Likes Received:
    6
    Joined:
    Sep 11, 2009
    Location:
    NewZealand
    Hi, whats the deal with speedo gears and changing to get a more accurate speedo?
    can you just change the driven gear I see they go from 18 to 22 teeth or do you need to change both gears at the same time?
    Mine is probably out by 12-13mph reads fast as I went from a 2.4 diff to 3.08 and have bigger rear wheels.
    I dont know what colour is in there now (was a 79 305 with 2.4 rear)
    Any tips or info appreciated
    Thanks
     
  2. 1978 Z/28

    1978 Z/28 Member Gold Member

    Messages:
    96
    Likes Received:
    22
    Joined:
    Jul 17, 2017
    Location:
    New Brunswick
  3. Gary S

    Gary S Administrator Lifetime Gold Member

    Messages:
    22,298
    Likes Received:
    720
    Joined:
    Apr 14, 1999
    Location:
    Bismarck, North Dakota
    Yes, you can change just the driven gear IF it gives you enough change. If the range of driven gears don't get you where you need to be, you have to change the drive gear too. However, the TH350 is fairly easy to change the drive gear because you can pull the tail shaft.
    You say 12-13mph fast...............At what speed? The wrong gears cause a percentage of error and it would be the same percentage at all speeds, not a fixed mph difference at all speeds. Once you know the percentage you are off, and know what driven gear you currently have in there, simple math will tell you if there is a different driven gear that will correct it. If not, you need both gears.
     
  4. JasonZE

    JasonZE Veteran Member

    Messages:
    516
    Likes Received:
    6
    Joined:
    Sep 11, 2009
    Location:
    NewZealand
    reading fast at 100 kph im showing just under 120 which is about 76mph at 65mph I have the speedo out currently doing dash lights etc so will get on the road in next few days to gps and then I will pull the drive out and see what gear is in there now and see if I can count the drive gear so i can use the calculators. (hoping the 2.4 diff means they had the smallest gear in there so I have some chance of getting it close only changing the dricen geatr)
    Cheers
     
  5. 70-camaro

    70-camaro Veteran Member

    Messages:
    2,216
    Likes Received:
    375
    Joined:
    Mar 25, 2010
    Location:
    marietta ohio
    This is from the TCI website for speedo gear colors. There should be no need to count the teeth.
    TH350 Speedometer Gears
    DRIVE GEARS ARE 7-8-9-10 teeth.
    Drive Gears Driven Gears
    7 teeth - pink 18 teeth - brown
    8 teeth - black 19 teeth - natural
    9 teeth - green 20 teeth - blue
    10 teeth - red or purple 21 teeth - red
    17 teeth - red 22 teeth - gray
    18 teeth - blue 34 teeth - light green
    19 teeth - yellow 35 teeth - orange/pink
    20 teeth - brown 36 teeth - white
    37 teeth - red
    38 teeth - blue
    39 teeth - brown
    40 teeth - black
    41 teeth - yellow
    42 teeth - green
    43 teeth - purple
    44 teeth - dark gray
    45 teeth - light blue
    Notes:

    [​IMG] Chevrolet transmissions use a 1" diameter steel driven gear housing located in the tailhousing on all two-wheel drive applications.
    [​IMG] Buick, Oldsmobile & Pontiac transmissions use a 2.078" diameter driven gear housing. Two housings, each made from either aluminum or plastic, are available for driven gears with 34 or more teeth. One fits gears from 34 - 39 teeth and the second one fits gears from 40 - 45 teeth.
    If pull the bullet out you can use a bore scope to see what color drive gear you have and not have to pull the drive shaft and tail housing.
     
  6. JasonZE

    JasonZE Veteran Member

    Messages:
    516
    Likes Received:
    6
    Joined:
    Sep 11, 2009
    Location:
    NewZealand
    thanks for that was helpful i have a purple drive gear 10 tooth and had a 19tooth driven. have gone to a 24 and new steel housing all installed and will get out tomorrow for a gps test run as raining today (its mid winter here) calculator said i needed a 23.3 so hope te 24 is close enough as it was the closest i could find without importing spmething at least its an easy job.
    cheers!
     
    70-camaro likes this.
  7. JasonZE

    JasonZE Veteran Member

    Messages:
    516
    Likes Received:
    6
    Joined:
    Sep 11, 2009
    Location:
    NewZealand
    well didnt quite go to plan now reads 6mph/10kph to slow which is bad as Im going through a modified vehicle certification check and it need to be accurate of read slight ly fast if anything not allowed to be slower. I will track down a 22 tooth gear and try that..
     
  8. 70-camaro

    70-camaro Veteran Member

    Messages:
    2,216
    Likes Received:
    375
    Joined:
    Mar 25, 2010
    Location:
    marietta ohio
    If I remember right 1 tooth on the driven equals 3 miles per hour. So you may jump to a 19 or 20.
     
  9. JasonZE

    JasonZE Veteran Member

    Messages:
    516
    Likes Received:
    6
    Joined:
    Sep 11, 2009
    Location:
    NewZealand
    i started at 19 so hope to try find a 22/21 gear locally
     
    70-camaro likes this.
  10. JasonZE

    JasonZE Veteran Member

    Messages:
    516
    Likes Received:
    6
    Joined:
    Sep 11, 2009
    Location:
    NewZealand
    21 tooth got it done. was an easy job that i had put off for years lol:)
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.