synchronizing speedometer

Discussion in 'The "Juice Box"' started by 36threewindow, Jun 3, 2019.

  1. 36threewindow

    36threewindow Well-Known Member

    I have a 1936 Buick series 40 3 window coupe originally had 233ci straght 8 with 3 speed manual trans. kept the original dash and Gauges but replaced engne & trans with a 1965 401 nailhead and mating thm400 trans with switch pitch.
    The speedometer at idle now shows 10 MPH and I'm sure is off at all speeds. I know there are gears on the trans h0ok up to speedo cable --- anyone know what size gear I need to change the trans to to get these in sync ?
  2. gsx455-4ever

    gsx455-4ever Gold Level Contributor

    If it says 10 MPH while sitting still I would say it needs to be removed and sent to a qualified repair person. It appears to be stuck at 10 mph and gears in the trans will be of no help with that condition .
    GranSportSedan likes this.

    BRUCE ROE Well-Known Member

    There is an anchor for the speedo needle hairspring, on mine it can be
    moved to read 0 MPH at rest. The speedo should be at the same
    inclination as installed. If this cannot be established consistently
    there may be dirt or other issues.

    Once that is fixed, measure the error, that is, what is the real speed
    when it reads 60 mph? Then check the trans speedo gears, figure
    the ratio, and use the error to calculate what ratio you really want.
    Find a new set of gears with that ratio, or add an external gear box.
    Bruce Roe

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