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Speedometer Cable and Driven Gear

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  • Speedometer Cable and Driven Gear

    I didn't see much on this subject, so I thought I would post a simple thread.

    My Speedo wasn't working, and I found the Cable was not turning.

    I pulled the cable from the housing at the instrument, and after inspection, the cable is fine. Needs a little more grease, but other than that, it is in tact and tensile strength is fine.

    So I pulled the Driven Gear. My results are below. Mounting Clip is held on by 7/16th bolt. New gear on the way.

    Geoff In Carlsbad
    1961 Thunderbird Convertible aka: Betty:cool:


  • #2
    See the Difference

    You can really see the difference in the condition of the teeth:

    Geoff In Carlsbad
    1961 Thunderbird Convertible aka: Betty:cool:



    • #3
      More Follow-Up with Questions

      I installed the new Speedometer Driven Gear, 18-tooth. Re-greased with graphite lubricant the cable, and voila, I have a working speedometer! Yeah!

      I received the car with 15" Rally Wheels. I believe these came with 14" wheels originally. I do not have those wheels, so that means that my speedometer is not really accurate. Seems to be about 5 mph faster than my application I check it against.

      Originally posted by edpol
      As far as speedometer accuracy is concerned, it doesn't matter what the wheel diameter is, as long as the tire diameter remains the same. For instance, if you have 215/75 -14 tires, and you switch to 205/75 -14 using the same wheels, the speedometer wil no longer be accurate. If you get 15" wheels, 215/75 -15 tires will work.
      So, is there any way to adjust the speedo to accommodate the larger tires? What was the spec for original tire size on these '61's?

      Not really important to me since I probably won't exceed 50 mph at any given time, but if there is a way to do that, I am all about making it right.
      Last edited by GeoffInCarlsbad; November 7th, 2015, 04:37 PM.
      Geoff In Carlsbad
      1961 Thunderbird Convertible aka: Betty:cool:



      • #4
        In case any cares....

        I've been doing some research and the consensus diameter for 8.00x14" seems to be 27.58". Many folks go with the 215 or 225R7514's. I went to the Coker site to confirm these. You can plug in the Year/Make/Model/Tire Size: ... d&q=800-14

        I also went to the Universal Vintage Tire site: ... ize=800-14

        Coker 8.00x14 - 27.58"OD x 4.58"W
        Coker 8.50x14 - 28.10"OD x 4.75"W
        Universal 8.00x14 - 28.67"OD x 4.95"W (but there were varieties of 800-14 with different diameters).

        I have 15" Rally Wheels so I have to figure out what tire sizes come close to the 27.58" diameter. I replaced my driven gear (18 tooth) recently and run about 5mph faster than actual speed up to 50 mph. After that, the differential gets larger as the small diameter turns faster than actual.

        Frankly, I don't need new tires right now, but when I do, I will try to figure out the closest match for a 15" wheel.
        Geoff In Carlsbad
        1961 Thunderbird Convertible aka: Betty:cool:



        • #5
          If the speedometer is reading faster then you are going, then I think you need to have a slightly larger gear (more teeth) on the driven side to slow it down. I think that would be the easy way to correct it. There should be a chart out on the net somewhere.

          Here is a site that compares tire size:

          I don't think they make a 225/75 -14
          Maybe a 225/70 - 14
          They are very similar in diameter.

          The old tires were 80 series, that means that 8.00-14 had a diameter of 26.79 inches or 68.06 centimeters.
          It's easy to calculate: 8 inches = 20.32 centimeters, 80 % of this means 16.25 centimeters (this is the height of the side of the tire)
          2x 16.25 =32.5
          32.5 + 35.56 (diameter 14 inch rim, 14 inches = 35.56 centimeters) = 68.06 centimeters or 26.79 inches

          The closest thing to the diameter of an 8.00x14 would be a 245/60R15 which is 26.6

          The speedometer on my Ranger is off and when I go past the electronic speed signs I see how it compares to my speed and I just compensate as I drive.