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And a Speedometer Question

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  • And a Speedometer Question

    So the speedometer in my 29 Roadster is overly optimistic, as in when it says I am going 50 Android Speedometer says no, it's actually 41. Then when 70 peeks around the corner Android says 52. I worried that the drive gears were wrong, but the odometer checks out about perfect. The speedo is a rebuilt my Dad bought when he restored the car in 2004, but unfortunately he is not around to ask if it ever was right. I called the place he bought it from and they said to expect a 5-10 mph inaccuracy, but I don't believe that and the error is more than that anyhow. As I recall the speedo in my other car (30 Town Sedan) used to oscillate but the average reading was pretty accurate (before the cable broke). Any opinions on what expectations should be, or what to do to fix what I have?

  • #2
    How accurate is the odometer? If the odometer is reading correctly, then the gearing for the speedo is fine. If you have a round speedo, there is an hair spring adjustment that can be made to correct the MPH readings. You will need to take the cover off and remove the inside to get to the hair spring on the top of the speed wheel. You will need to hold the speed wheel in one hand so the wheel will not turn then turn the center of the spring counter clockwise to speed it up or clockwise to slow the speed down.

    Sorry for the post on the round speedo, just reread your post and it's for a 29, not a late 30-31.

    This link may help with the oval speedometer:
    Last edited by AL in NY; 02-07-2018, 08:54 PM.


    • #3
      Are you running 21" tires? Is the rear ratio stock? Certain things can come into play on speedo accuracy. The vendors sell different driven gears to compensate for non stock rear ratios. Here are a couple links and you can call Mark's to get some info about your speedo head
      3~ Tudor's & 1~ Coupe
      Henry Ford said,
      "It's all nuts and bolts"
      "Start by doing what's necessary; then do what's possible; and suddenly you are doing the impossible."

      Mitch's Auto Service ctr


      • #4
        Possibly you should call Harry Wardburton and see what he would charge to just do a calibration. I do not have his number in front of me. Have sent several to Harry over the years and have always been happy with his work. Rod
        "Much of the social history of the Western world, over the past three decades, has been a history of replacing what worked with what sounded good." Thomas Sowell


        • Mitch
          Mitch commented
          Editing a comment
          No known website: Harry Warburton (252)-258-1530/(252)-752-4098 

          This is off the website list... I'm not sure if the info is still valid

      • #5
        Thanks for the help! Like I said the tires and gearing are not in question because the odometer is accurate. I may tear into it and try some of the calibration tips from the link Al sent above.


        • #6
          Slightly off topic, but I have an F150 gearbox in my car and at 30mph the speedo reads 37mph and at 62mph it reads 75mph. At 62mph the motor is doing 1670rpm.
          Recently we were in a hurry to make up time and the speedo sat on 75mph but I noticed it was doing 2200rpm ( I have a Tiny Tach installed)
          Something seemed wrong and it was only later that I discovered that 75mph is the highest speed the Model A Speedo goes to.
          In reality we must have been doing 70mph+ (I should add that normally I have a Sat Nav in use as a speedo and on this occasion I had not set it up)
          Last edited by wensum; 02-08-2018, 03:14 PM.


          • DaWizard
            DaWizard commented
            Editing a comment
            What you really need to check is the Odometer readings. If it is off you need to figure how much and you can change the gears to adjust that, and when you get that on, the internals of the speedo can be adjusted to get the mph close. And yes, the speedo only goes to 75mph

        • #7
          Thanks DaWizard. What do you mean by changing the gears? the F150 gearbox originally had an electronic drive unit for the speedo, but I found that a Ford Falcon speedo cable fitted directly into the gearbox drive and had a cable made up with the Falcon fitting at the gearbox end and the Model A fitting at the other end. it all works but the speedo is obviously reading somewhat fast!


          • #8
            It sounds to me like your trans has a gear drive speedo pickup, not electronic. If this is so, you can change the gears to change the odometer reading. If as you say it is magnetic or electronically calculated, it is beyond me and I suggest you find someone who repairs speedometers in your area and have then see about fixing it, I can tell you that the Model A Ford speedometer likes 1000 revolutions to the mile.
            You wana look waaay far up da road and plan yer route because the brakes are far more of a suggestion than a command!


            • #9
              Wensum: If you are using the speedo drive on the F150 transmission, you must be using open driveshaft, not the torque tube. If that's true, you need to find a F150 transmission speedometer drive gear that will close match your Model A rear axle ratio.


              • #10
                " but the odometer checks out about perfect"

                Forget the drive gear since he said the odometer is about perfect.
                It's inside the speedometer, and the magnets or hairspring most likely need to be adjusted.


                • #11
                  Tom, was answering "Wensum" about his speedometer with the F150 transmission.


                  • #12
                    Originally posted by AL in NY View Post
                    Wensum: If you are using the speedo drive on the F150 transmission, you must be using open driveshaft, not the torque tube. If that's true, you need to find a F150 transmission speedometer drive gear that will close match your Model A rear axle ratio.
                    Yes, I am using an F150 transmission with an open drive shaft. Both the F150 Gearbox and the T5 gearbox a friend is installing have take-offs for electronic speedos, but when you withdraw the unit you find a gear on the end of it and this is identical to a Ford Falcon (and other Fords I presume) cable driven speedo fitting. T Falcon speedo.jpg hey are interchangeable. The cable gear appears to be some kind of plastic.
                    The attached photo shows the electronic take-off and the two gears are for a cable driven speedo1960-1970 SPEEDOMETER DRIVEN GEARS


                    • #13
                      Wensum: are there different gears available (ie number of teeth) for the Falcon speedo drive. From what you said, your Model A speedometer is reading about 21% high, so you need to find a gear that has about 17% MORE teeth then the one you have. That should get you close.


                      • #14
                        To answer post 1, as Larry posted in 14, Nick sometimes has sales on rebuilds , etc.. Another option which worked for our F150 tranny was to get a ratio adapter (on line). We used our cell phone/ GPS to calibrate the speed compared to the speedo reading and did simple math to get the ratio needed. Ours came with some cable ends to use for the drive gear input.
                        Last edited by plyfor; 02-10-2018, 09:30 PM.


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