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Speedometer (Revolutions Per Mile) ???

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  • Speedo / Tach / Gauges: Speedometer (Revolutions Per Mile) ???

    I did a search and found two vague responses. So, I need to ask... :

    Does anyone know the revolutions per mile a 1964 Studebaker speedometer would have? I'm told it is often printed on the speedometer face..., but below the bezel. So, if anyone has one taken apart perhaps you could look.

    A common number is 1000 revolutions per mile, but not all speedometers hold to this. Without this number being correct, the many calculators I have found become inaccurate, - regardless of providing the tire size (diameter) and axle ratio.

    The application is a 700R4 behind a Chevy 350. The rear ratio is 3.07. The tire size is 225-60-16 (26" diameter).

    Thanks, Tom
    '64 Lark Type, powered by '85 Corvette L-98 (carburetor), 700R4, - CASO to the Max.

  • #2
    Most speedometers are calibrated so 1000 rpm (cable rpm) is 60 mph. RPM (minute)

    Your speedometer's cable speed is affected by tire diameter, and rear gear.
    Run your car at a GPS speed of 60 mph.
    Read your speedometer.
    Divide the larger into the smaller number. (Actual speedo number and GPS speed number of 60 mph)
    That will be the percentage of difference you need to adjust to.
    (The smart math guys can jump in here and correct the math)
    HTIH (Hope The Info Helps)


    Get your facts first, and then you can distort them as much as you please. Mark Twain

    Note: SDC# 070190 (and earlier...)


    • #3
      Thank you for your reply. Unfortunately I have never driven this car, much less able to now without the trans swap complete. I think I'll just go with the 1000 rev's per mile get the gears and see where I'm at.

      '64 Lark Type, powered by '85 Corvette L-98 (carburetor), 700R4, - CASO to the Max.


      • #4
        I dunno about a '64 but can tell you a '53-'54 speedo is 1000 rev per mile on the odometer. I have 1st hand (literally counted it while turning shaft) knowledge.

        Jeff in ND


        • #5
          As DEEPINHOCK points out, make the final correction using a GPS as reference. I just corrected my speedo reading on a '64 Daytona. According to the chart for a 3.07 rear axle ratio and 205-75R15 tires (6.70-15 equivalent), the speedo pinion gear should have 18 teeth for an automatic trans. However, it required a 20 tooth gear to match the GPS. I have to conclude that the drive gear is a 7 tooth rather than the correct 8 teeth. Probably the transmission has been swapped by a PO on this "low mileage" car. No matter, the speedometer reads correctly now.

          BTW, I verified that the ratio is 3.07 and not 3.31.