Drive gear TH350

Discussion in 'The Hides' started by Tripple7, Nov 19, 2020.

  1. Tripple7

    Tripple7 Well-Known Member

    Planning on doing following change to my '71 350-2 Skylark Custom : from 225/60R14 to 245/60R15 front, 275/60R15 rear..
    Do I have to change the drive gear as well or only the driven gear?
    What was the drive gear for a standard TH350 and 2.56 rear?
     
  2. DasRottweiler

    DasRottweiler -BuickAddict-

    Use the calculator here to figure out what gear changes to drive and driven gears you will have to make based on tire height , gear ratio etc.


    https://www.tciauto.com/speedometer-gear-calculator

    If your changing tire height only, not gear ratio, changing the driven gear will get you where you need to be...

    JIM
     
  3. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    I don't think there is a standard DRIVE gear. You'll need to open it up and count the teeth through the DRIVEN Gear sleeve hole. The rear tire is all that matters. The 275/60R-15 is 28" tall The stock tire size is about 26.6", so your current 225/60R-14 are 24.6" tall, so your speedometer is way off now.

    If you jack the back of the car up high, and pull the sleeve out, you can turn the driveshaft with the trans in neutral, and count the DRIVE gear teeth. The gear is helical so it isn't the easiest, but it is doable. Mark one tooth with some white grease and turn the driveshaft. Do it a few times so you get it right. You just need the three numbers. Tire height (28"), gear (2.56), and the number of teeth on the DRIVE gear. Then just pop it into the formula to get the DRIVEN gear needed.

    https://www.tciauto.com/speedometer-gear-calculator
     
  4. Tripple7

    Tripple7 Well-Known Member

    Thanks Larry, wanted to avoid this procedure initially.
    Just couldn't find a specific number as for the TH400 in the manual.
     
  5. Tripple7

    Tripple7 Well-Known Member

    Yeah, only changing Tire Size. Thanks Jim
     
    DasRottweiler likes this.
  6. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Really no way to avoid it. If you get the back of the car up, you can avoid as much fluid loss as possible. You'll have to pull the sleeve anyway to change the DRIVEN gear. Might as well be sure.
     
    Tripple7 likes this.
  7. DasRottweiler

    DasRottweiler -BuickAddict-

  8. Tripple7

    Tripple7 Well-Known Member

    Ok, just finished the Job.
    Ok, just finished the Job.
    I have a 18 tooth ( green ) drive gear and a beige 35 tooth driven gear..
    According to the computation I will need a 33 tooth driven gear for my 275/60R15 Setup right? 20201121_144440.jpg 20201121_144414.jpg
     
  9. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Yes, 33, but no such thing. 34 teeth is the closest. You can change the drive gear easy enough if you want.
     
  10. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    19 and 35 will work.
     

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