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Transmission Speedo Gear 74 Lesabre

Discussion in 'The "Juice Box"' started by Detrailers, Mar 17, 2016.

  1. Detrailers

    Detrailers Well-Known Member

    Detrailers, Mar 17, 2016
    #1
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    I have a 1974 Buick Chassis manual so I can look up what DRIVEN gear came stock in your THM400, BUT, first, what rear axle ratio do you have, and are you running the stock size tires? Also, does the car still have the numbers matching transmission, in other words, has the transmission been replaced. Should be a 74-BC or 74-BT code on the tag located on the passenger side of the transmission.

    Yes there are 2 gears, the DRIVE gear located on the output shaft of the transmission, and the DRIVEN gear that goes into the sleeve that installs in the transmission.

    If things have been changed, then you have to know all the specifics to figure out which gear you need.
     
    LARRY70GS, Mar 17, 2016
    #2
  3. BRUCE ROE

    BRUCE ROE Well-Known Member

    If you know the car axle ratio and the rolling diameter of your tires, its possible to calculate the
    exact trans ratio to give you the speedo cable 1001 turns per mile required. However the tire
    rolling diameter isn't exactly the same as a perfectly round unloaded tire, and isn't always known.
    In fact it can vary between brands of the same sizes.

    The more straightforward method avoiding the above problems, is just make an accurate
    measurement of the error of your current setup, I prefer mileage & odometer since it
    eliminates the need for constant speed. Once you know the ratio of displayed mileage to
    actual, simply change your trans gear ratio to one multiplied by the error ratio, and you
    will be dead on. The TH400 has a speed driven gear available from 34 to 45 teeth, and a
    drive gear typically 17 to 20 teeth. Sometime just changing the driven gear will get there,
    sometimes both, and sometimes an external speedo cable gearbox will be needed to get
    the required overall ratio. Bruce Roe
     
    BRUCE ROE, Mar 17, 2016
    #3
  4. Detrailers

    Detrailers Well-Known Member

    Wow guys thanks for the info. My head is spinning lol. I found out the stock one for my car is a 41t and found one on eBay. Im going to try that since I havent a clue what my final drive ratio is. It is the stock tire size however since the hasnt been donked lmao
     
    Detrailers, Mar 17, 2016
    #4

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