4 speed speedometer help

Discussion in 'U-shift em' started by mll1446, Jun 17, 2020.

  1. LARRY70GS

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

    Not all THM350 or 400's for that matter had the same DRIVE gears. They were different. It also varied by year. In 1972, the BB, and BS 400 had a 16 tooth DRIVE gear. The BC, BT, and BU 400's(All big cars in 72) all had 18 tooth DRIVE gears. My 1967 BT switch pitch had a 17 tooth DRIVE gear.
     
  2. buick64203

    buick64203 Just plum crazy Staff Member

    That is a .785 adapter for a car with 3.64s. Non AC car.

    That adapter is unobtainable. Dont even bother searching for one. Doesn't exist.

    You can remove it and use the gray gear. With G60s you should be good to go.
     
  3. mll1446

    mll1446 Active Member

    Ok update disconnected reducer and put cable into trans and car speedometer works but 15 mph fast
     
  4. mll1446

    mll1446 Active Member

    Thanks
     
  5. BuickV8Mike

    BuickV8Mike SD Buick Fan

    Sounds like you need less teeth, is that correct? Its linear as well.
     
  6. LARRY70GS

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

  7. buick64203

    buick64203 Just plum crazy Staff Member

  8. LARRY70GS

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

    I'm talking about the OP. I wouldn't care as long as it works. What say you Mll 1446?
     
  9. LARRY70GS

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


    No, more teeth, to slow it down.
     
  10. buick64203

    buick64203 Just plum crazy Staff Member

    He doesn't need it. I put the gray gear in mine as per Dave Johnsons recommendation and it works like a charm. Within 3-4 mpg against my gps speedo on my phone
     
  11. LARRY70GS

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

    How many teeth on that Gray gear?
     
  12. 65GSConv4sp

    65GSConv4sp Well-Known Member

    OK, that makes sense. Then the chart is too general regarding automatics to know what the driven gear should be, as it doesn't define automatics further than THM350 & THM400. Oh well. Thanks for the enlightenment. Appreciate the good feedback.
     
  13. LARRY70GS

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

    The chart would probably only be good for 1972 cars. I have, 1967, 1970, 71, 72, and 74 Chassis manuals. The 72 manual is the only one that shows the DRIVE gear tooth count.
    SpeedoDriveGearChart.jpg
     
  14. buick64203

    buick64203 Just plum crazy Staff Member

    The assembly manual is what your looking for as far as a chart

    Gray gear has 22 teeth.
     
  15. LARRY70GS

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

    That would mean the DRIVE gear has 8 teeth? Does the assembly manual show that?

    1D.JPG
     
  16. Mart

    Mart Gold level member

    Take that big nut off and it will look like this.
    This is out of a 4 spd muncie, 68GS 350. This gear has 20 teeth. The rear gear that was in the GS was a 390 ratio, which is the trailering option ratio. FYI, Not sure if this gear jives with the 390 & speedo as it was a parts car from original owner. He ran it at the strip.
     

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