Speedometer calibration
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Speedometer calibration
Does anyone have any tips to calibrating the speedometer in a 1966 Ford F250 Custom Cab truck(352 motor 4 speed manual transmission)? I calibrated it a while back by trial and error and after taking the cluster apart about 5 times finally got it close to accurate. Now it is in need of calibration again. I already adjusted it once and it made it not work at all. Is there a particular place that the needle should sit when idle or something?
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Its not working at all now. Before I adjusted it it was running at around 20 when the actual speed was 55. I probably adjusted it the wrong way. Also it seems like cold weather badly effects the accuracy, which is probably normal but it will make it not work at all if the temp is below 20 degrees or so until it runs for a while.
Logistics Pro
Joined: Jun 2018
Posts: 3,554
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I'm not saying this is the one but there are ratios and applications to correct your speedometer.
https://www.ebay.com/itm/Stewart-War...QAAOSwN35eUAOE
https://www.ebay.com/itm/Stewart-War...QAAOSwN35eUAOE
Fleet Mechanic
Joined: Sep 2016
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From: Southern Maryland
The first thing you need to do is figure out what driven gear you should have. The formula looks like this:
drive teeth x axle gear ratio x tire rev per mile/ 1001 = driven teeth
The NP 435 which is what you should have for a trans has 7 drive teeth. A typical tire used on a 16" F250 rim is the P225-75-16. Using Tire Racks specs for a BF Goodrich tire that size, the revs per mile are 710. You can't use straight math for revs/mile because the loaded/rolling tire circumference is less than the unloaded static circumference, so use the published specs from the manufacturer. You don't state the rear axle ratio, but lets use 4.10 for an example.
7 x 4.10 x 710/1001 = 20.35 or rounding off to whole numbers, 20 teeth. That is a black gear with a RH pitch (////) p/n C1DZ-A.
drive teeth x axle gear ratio x tire rev per mile/ 1001 = driven teeth
The NP 435 which is what you should have for a trans has 7 drive teeth. A typical tire used on a 16" F250 rim is the P225-75-16. Using Tire Racks specs for a BF Goodrich tire that size, the revs per mile are 710. You can't use straight math for revs/mile because the loaded/rolling tire circumference is less than the unloaded static circumference, so use the published specs from the manufacturer. You don't state the rear axle ratio, but lets use 4.10 for an example.
7 x 4.10 x 710/1001 = 20.35 or rounding off to whole numbers, 20 teeth. That is a black gear with a RH pitch (////) p/n C1DZ-A.
Lead Gopher
Joined: Aug 2012
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From: Seattle
If the speedometer is currently not working and you have done a proper test on it per the shop manual then I'd think at this point no change to drive or driven gears or adapters will make any difference. You mentioned calibrating it by trial and error. I assume you have a sweep speedometer since you say 1966 Custom Cab. Now, I'm inferring that which I don't love to do, so you will have to tell me if I'm wrong.
Here's the back side of a sweep speedometer. Can you tell us what you were altering here in order to alter the calibration?
The speedometer cable drives magnets which creates an Eddy current that drives the speedometer pointer through that round circle looking item. Calibration of the speedometer head itself is typically done by altering the magnetic strength of the magnets.
Here's the back side of a sweep speedometer. Can you tell us what you were altering here in order to alter the calibration?
The speedometer cable drives magnets which creates an Eddy current that drives the speedometer pointer through that round circle looking item. Calibration of the speedometer head itself is typically done by altering the magnetic strength of the magnets.
Lead Gopher
Joined: Aug 2012
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From: Seattle
Here's a better explanation.
Flannery Speedometer Repair » Dr Mike Flannery
Also good reading.
http://www.magnetic-speedometer-repa...leshooting.php
Flannery Speedometer Repair » Dr Mike Flannery
Also good reading.
http://www.magnetic-speedometer-repa...leshooting.php
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If the speedometer is currently not working and you have done a proper test on it per the shop manual then I'd think at this point no change to drive or driven gears or adapters will make any difference. You mentioned calibrating it by trial and error. I assume you have a sweep speedometer since you say 1966 Custom Cab. Now, I'm inferring that which I don't love to do, so you will have to tell me if I'm wrong.
Here's the back side of a sweep speedometer. Can you tell us what you were altering here in order to alter the calibration?
The speedometer cable drives magnets which creates an Eddy current that drives the speedometer pointer through that round circle looking item. Calibration of the speedometer head itself is typically done by altering the magnetic strength of the magnets.
Here's the back side of a sweep speedometer. Can you tell us what you were altering here in order to alter the calibration?
The speedometer cable drives magnets which creates an Eddy current that drives the speedometer pointer through that round circle looking item. Calibration of the speedometer head itself is typically done by altering the magnetic strength of the magnets.
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Mountain Pass
Joined: Jan 2003
Posts: 131
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From: Vernon, Arizona
When you spin the input shaft now, the needle does not move? If so, there may be some crud or corrosion on the shaft. The spinning magnet may not have enough power to move the needle to overcome the resistance on the shaft. The second link in post 7 by TA455HO is a great trouble shooting guide explaining this. Since the wheel and the spinning magnets are completely disconnected except by magnetic forces you are not actually calibrating or making changes to anything. I'm guessing the last time that you did this, you had freed up the speedometer shaft some and it worked for a while. Look for corrosion on the spring and everything else as that might give you a clue. The only calibration that is likely to be an option up on that end would be modifying the spacing of, or strength of the magnets, or changing the spring.
Worst case now may be that the leaning on the needle as you have tried to fix it, you may have bent something that is now binding up as well.
Worst case now may be that the leaning on the needle as you have tried to fix it, you may have bent something that is now binding up as well.
Lead Gopher
Joined: Aug 2012
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From: Seattle
At this web link is an article from a newspaper about The Speedo Shop and it describes more of the intricacies of calibrating the old magnetic speedometers like our trucks use.
https://www.sandiegouniontribune.com...htmlstory.html
https://www.sandiegouniontribune.com...htmlstory.html
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