Calibrate Speedometer

Hugh in NC

Hugh in NC

Member
On my 1930 ccpu the speedometer runs about 4 miles per hour faster than the truck is going; it registers 45 but the gps says it is going 41. I took off the turtle this afternoon and it has 9 34 and then under that 19 cast into the turtle housing. I understand that to be 19 teeth, 3.78 rear end. If I change to the 18 tooth gear, will the speedometer slow down and by how much?

Thanks,
Hugh
 
Before you change the gear, do a check and see if the odometer is recording a mile for each mile driven. You can usually see mile posts on some roads for an odometer check. If the odometer is correct than an internal adjustment must be made to adjust the speed cup so it indicates correctly. If the odometer is off proportionately to the slower speed indicated then it is an incorrect gear for the rear end ratio or incorrect tire size on the car.
 
I had 2 of the same ones that were off by almost 10mph. Sent them to Leonard Nichols Jr. and he did a great job of adjusting them. They are now within 1 mph. He is very reasonable. He usually has an ad in the swapmeet section. Very quick turnaround time too. He got mine back in 2 days. He is somewhere in NY.
 
Can a person get an A speedometer any more accurate that what this one is? Also I fine it very hard to read an accurate GPS speed also.
 
160B-The odometer works but I didn't think to check accuracy. The internal adjustment is something that cannot be done at home?

Hicktick-If I just leave it alone, I might have the fastest A around.

Hugh
 
If the odometer is accurate, then the problem is most likely with the speedometer itself. After 80 years or so, the magnet and or the hairspring lose some of their strength. I find that most of the speedometers that I rebuild need their magnets recharged. If the magnet is too weak it will not pull the speedcup high enough to show the right speed. The easiest way to check and adjust the calibration is on a calibration machine, balancing the hairspring against the magnet strength to get the best accuracy.
 
Hugh in NC said:
160B-The odometer works but I didn't think to check accuracy. The internal adjustment is something that cannot be done at home?

Hicktick-If I just leave it alone, I might have the fastest A around.

Hugh
Click to expand...


Hugh

Take a look at the below article and you can make your own decision to tackle the calibration.

A fellow club member and I put on a seminar at the MAFCA National Meet in Vancouver in 2010 on Servicing Model A Ford Speedometers. There is a lot to learn in the repair and servicing of speedometers.

Below is a link to an article on Model a Ford speedometer that covers the disassembly, cleaning, repairing, installing decals, lubrication, calibration, and assembly of the familiar oval speedometers manufactured by Stewart Warner.

The round speedometer is similar in the calibration and actually is easier to work on.

MODEL A FORD SPEEDOMETERS. Handouts courtesy of THE BEAVER CHAPTER, MAFCA Portland, Oregon.
Article and Sketches by Gordon Fischer. Copyright 6-12-94

http://www.modelahouse.com/tech/speedometer/fischer.html

You will need a to run the Speedometer at a specific RPM to check the accuracy and determine what adjustments need to be made to correct the speedometer to an accurate reading see the table below.

RPM to MPH table

RPM MPH
167 10
500 30
750 45
1000 60
1250 75
1500 90

good luck!!!!

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'28 CCPU, my first car, had a defunct speeomometer FOREVER. I followed Chief @ 50 MPH & I discovered it was charging 10 AMPS! I didn't need NO stinkin' SPEEDO! Bill W.
 
Hugh;

I had a similar problem. I did as 160B suggested and checked the odometer first against a GPS. I found that it was dead on. I removed the speedometer and removed the cover. There is a watch like spring that controls the amount of tension the movable part of the speedo is subjected to. The spring is held to a post with a very small wedge. I removed the wedge (with hands shaking), put more tension on the spring and replaced the wedge (with hands shaking). I then replaced the cover and reinstalled the speedo and went for a drive with the GPS running.

I did this about two or three times to get the speed close to the GPS at 45 mph.

It has wondered a bit in the meantime (I did this several years ago). I think that the accuracy of the speedo is temperature sensitive as well, but others with more knowledge than me can chime in on that.

Good luck. Dick
 
i read the tech that 160b posted on disassmby of the unit my question is do you have to remove the face to remove the back i tryed to remove the back and it would not move i did not force it becouse it works fine i just want to set the odem to 0 for my rebuilt motor
kevin
1930 model a tudor
1923 model t
 
kwisor said:
i read the tech that 160b posted on disassmby of the unit my question is do you have to remove the face to remove the back i tryed to remove the back and it would not move i did not force it becouse it works fine i just want to set the odem to 0 for my rebuilt motor
kevin
1930 model a tudor
1923 model t
Click to expand...
Dog:cool: here, let me help you, it's spelled:
O-D-U-M-O-M-A-H-M-E-T-E-R, it's easy! All together now, ODUMOMAHMETER What is it anyhow? Buster T.:cool:
 
kwisor said:
i read the tech that 160b posted on disassmby of the unit my question is do you have to remove the face to remove the back i tryed to remove the back and it would not move i did not force it becouse it works fine i just want to set the odem to 0 for my rebuilt motor
kevin
1930 model a tudor
1923 model t
Click to expand...

Hope this helps with the removal of the speedometer from the case. These photos are for an oval Speedometer, but the removal process is the same for the round Speedometer.

Seal Removal
Be careful not to pry against the speedometer case. The post is pot metal and an integral part of the frame. It is brittle. Pry against the top of the post carefully
picture.php

Bezel Removal
Use a soft drift (brass, aluminum, phenolic) Support the speedometer to protect the trip stem.

picture.php
 
Dick Deegan said:
Hugh;

There is a watch like spring that controls the amount of tension the movable part of the speedo is subjected to. The spring is held to a post with a very small wedge. I removed the wedge (with hands shaking), put more tension on the spring and replaced the wedge (with hands shaking).

Good luck. Dick
Click to expand...

There is also an internal plate for additional adjustment. I usually leave the hair spring alone and only adjust it so the speed cup returns to zero in a smooth manner.


Calibration - Note the open gap on the Speedometer on the right, now compare to the Speedometer on the left the sliding plate is moved to cover the "air gap". This adjustment affects the strength of the magnetic field and thus the speed of the speed cup. When the gap is open the magnetic field is the strongest and this would make the indicated speed increase. It should be noted that the speed cup is aluminum and the magnetic field creates an eddy current which causes the speed cup to move.

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hello
thanks for 160b's photos on how to remove the cover to take speedometer apart i was able to take mine apart and reset my odumomahmeter back to zero i put it back in and did a test drive and all is working well
thank you
kevin
1930 model a tudor
1923 model t
 
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Thanks 160b for all the information. As I stated in my initial post above, my speedometer was registering too fast. I took out the speedometer and disassembled according to 160b's instructions. The slot and rivet was centered. I moved it all the way clockwise to look like the illustration on the left above. Reassembled and went for a test drive. It slowed the indicated speed down by 5 mph. The speedometer is now 1 mph slow, I went too far with the adjustment. I am going to leave it alone for now, it's a lots closer that it was. If I ever have it apart again I will make a minor adjustment counterclockwise. Thanks to all for the help and comments.

Hugh
 
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