1955 Speedometer Accuracy Adjustment

[JimLeanna55Bird]

So my speedo is about 9 miles on the high side. When I sit at idle not moving the speedo is on 9 mph. When driving it stays about 9 miles on the fast side. My gps will say I am going 35 MPH and speedo reads 44 mph. What is really interesting is that when car is not running the speedo drops to zero!!!
Any thoughts on adjustments that can be made?

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[Tech writer 60]

Using a needle puller. Remove needle and zero. Only practical way to zero a Mechanical simple instrument.

 

[doug7740]

The speedometer needle is pressed on to a knurled shaft and the shaft is joined to the metal cup. As the speedometer cable spins the magnet around the metal cup, centrifugal force causes the cup to move resulting in the movement of the speedometer needle.





When slowing down or stopping, the return spring allows the speedometer needle returns to zero. The centrifugal force that causes the movement of the speedometer needle must also overcome the tension of the return spring.



Look closely and you will observe that the return spring is pinned to the housing at the outer end. To calibrate the speedometer gently rotate the pin location to increase or decrease the tension on the spring.



I believe that the problem with your speedometer is a lack of lubrication where the speedometer needle shaft goes through the brass bushing. The only way to repair this is to remove the speedometer so it can be cleaned and properly lubricated. There is a lubrication hole on the back of the speedometer head that you can put a few drops of lubricant on the wick through the hole, but that lubricates the shaft that spins the magnet around the cup and will not lubricate the brass bushing where the speedometer needle shaft passes through.

doug7740
1955 Thunderbird Blue

 

[Frankie the Fink]

I would first ask, "....has the speedo EVER read correctly ?"
Great pics and explanation BTW.

 

[JimLeanna55Bird]

Great explanation on how it works! Thanks. I don't know about being accurate before as I have only the had the car about 3 months. I need to think about how much is 9 mph worth in regards to time and hassle on pulling the speedo or changing the gear in the trans. Thanks everyone for the insights.

 

[doug7740]

Half the fun of owning a classic car is doing the work yourself.

doug7740
1955 Thunderbird Blue

 

[Frankie the Fink]

Calibrating a vintage speedo requires special equipment and is just money if you can get it out of the dash to send off for the work then that's the big pain IMO I don't know if T-Birds are like GM products but I wonder if they use a transmission drive gear that may be wrong or could be changed...

 

[doug7740]

CASCO sells four different driven gears for calibrating the speedometer, however since the problem is the speedometer needle stays at 9 MPH and doesn't return to zero, replacing the driven gear will not correct the problem. Removing the speedometer from the dash and repairing it is not as difficult as you may think.

doug7740
1955 Thunderbird Blue

 

[seatramp]

I had the opposite problem with my speedo being 10 mph slow, thanks to Doug the guys here at the forum I figured out the rear axel (and thus the gear ratio) had changed. Swapping out the speedometer cable gear brought it within 2 mph and then used the return spring adjustment to bring it into spec. Hardest part of the fix was getting the speedometer cable off.

 

[Pink T-Bird]

So my speedo is about 9 miles on the high side. When I sit at idle not moving the speedo is on 9 mph. When driving it stays about 9 miles on the fast side. My gps will say I am going 35 MPH and speedo reads 44 mph. What is really interesting is that when car is not running the speedo drops to zero!!!
Any thoughts on adjustments that can be made?

My best guess (emphasis on guess) is that the tension spring in the speedometer is fatigued or otherwise not properly tensioned. There must be enough electromechanical current once the ignition is turned on to interact with the magnet and overpower the spring pulling it to 9 mph.
Time to remove the speedometer and have it serviced and calibrated. Make sure you tell them about the 9mph offset when ignition turned on.

 

[doug7740]

This 1950's technology, the speedometer needle moves by the centrifugal force of the magnet spinning around the metal cup. There is no electromechanical current involved and the ignition does not have to be turned on for the speedometer to operate.

doug7740
1955 Thunderbird Blue

 

[Tom Alpers]

The spinning magnet creates eddy current in the metal cup. Its somewhat like an very weak electric motor trying to start up. The spring keeps it from actually spinning. The faster the magnet spins the higher the eddy current and the more force the "motor" effect produces which makes it turn more against the resisting force of the spring and it reads a higher speed.