Speedo Drive / Driven Gears

Discussion in 'Transmission & Driveline Topics' started by 1981zapper, Sep 13, 2019.

  1. 1981zapper

    1981zapper Veteran Member

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    Since day 1 of owning the car, my speedo was never accurate, Originally my car came from the factory with 3.42 gears/ turbo 350 (non-lock up converter) Transmission and 225/70/15 tires (27.4" tall)

    my current set-up is;
    - 3.73 gears outback
    -Turbo 350 (non-lock up converter)
    - 275/40/17 Tires (25.7" tall

    Today I removed the Speedo bullet on the side of transmission and I have 22 Teeth on the driven gear, The Drive gear is greenish color any idea how many teeth it would be??

    IMG_1567.jpeg
    IMG_1563.jpeg
     
  2. Twisted_Metal

    Twisted_Metal Administrator Staff Member Lifetime Gold Member

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  3. 1981zapper

    1981zapper Veteran Member

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    so with a 9 Drive gear, I will need a 25 driven gear?
    current set-up is;
    - 3.73 gears outback
    -Turbo 350 (non-lock up converter)
    - 275/40/17 Tires (25.7" tall
     
  4. Twisted_Metal

    Twisted_Metal Administrator Staff Member Lifetime Gold Member

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    Probably not.

    A 25 tooth driven gear requires a different diameter drive gear.:(

    Try calculating for your driven gear.
    If that doesn’t give you a workable combination, keep trying.

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

    You may need to change both gears or buy a speedometer ratio adapter.

    Several ways to accomplish the same thing here.

    Good Luck!
     
  5. tom3

    tom3 Veteran Member

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    On the turbo trans changing the driver gear can be done without a lot of trouble. Several sites that will calculate the gears needed.
     
  6. 1981zapper

    1981zapper Veteran Member

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    i used an online calculator and a 25 driven and 9 drive gear will work for my combo

    now i need to find out if a 25 tooth and 9 tooth will mesh with each other

    [​IMG]
    https://www.summitracing.com/int/parts/oer-3860348/overview/
     
  7. Twisted_Metal

    Twisted_Metal Administrator Staff Member Lifetime Gold Member

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  8. cali cruzer

    cali cruzer Veteran Member

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    You could always look for a speedometer ratio reducer/adapter
     
  9. rchydzik

    rchydzik Veteran Member Lifetime Gold Member

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    You are looking for speedometer rpm of 1000 at 60 mph. So, with 27.4 inch tires, output shaft rpm at 60 is 2516. The speedo gears ratio that down to 1000.

    With 22 teeth driven, you need (1000rpm/2516rpm)*22teeth = 8.74teeth on the drive gear. You can vary each gear to get closer to round numbers.

    1000 rpm = 2516rpm*(drive gear / driven gear)
    8/20 would work well.
     

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