Driven gear

Discussion in 'The "Juice Box"' started by '72 Skylark Custom 4bbl, Jul 3, 2020.

  1. '72 Skylark Custom 4bbl

    '72 Skylark Custom 4bbl Well-Known Member

    Hey guys. I’d like to know what I need to do for my speedo driven gear.

    I stayed with my TH350. Changed out the rear from 3.08 to 3.42 gears & went from 225/70 14’s to 275/60 15’s. I want my speedo to be as accurate as possible.

    Can I just change the driven gear or will I need to change the drive gear as well?

    Kyle
     
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    The only thing missing is the number of teeth on the DRIVE gear. Need to know that to answer your question.
     
  3. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    With a P275/60R-15 tire (28") and a 3.42 rear gear, some DRIVE/DRIVEN gear combos that will work are 15/37, 16/39, 17/42, and 18/44.
     
  4. '72 Skylark Custom 4bbl

    '72 Skylark Custom 4bbl Well-Known Member

    Thanks Larry.

    Can I pull the driven gear to count the drive gear teeth and or do I need to drop the driveshaft & pull the tail shaft? Or can I go by the color of the drive gear?

    Kyle
     
  5. '72 Skylark Custom 4bbl

    '72 Skylark Custom 4bbl Well-Known Member

    Thanks again. Hardest part seems to be finding out which drive gear I have.

    Kyle
     
  6. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    It's a pain, but the best way is to get the back of the car up in the air as high as possible. Pull out the gear sleeve. You'll be able to see the DRIVE gear. Put the transmission in neutral, and turn the driveshaft while counting the teeth. It's a helical gear so it's not easy, but not impossible either. Mark one of the teeth with grease or paint and turn the driveshaft. Go slow and do it a few times to verify.

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

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