Speedometer question - Technical
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Elder User
Joined: Jan 2014
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From: Chillicothe, IA
Speedometer question - Technical
Ok, so during a speedometer problem post there was a link to a article on how speedometer works. I had to read it because I am having some weird problems with mine.
Symptoms. It changes how much it is off. I have larger than original tires, so if I have read correctly, my speedometer should be reading slow. It doesn't, it reads fast and anywhere from 5 mph to 20 mph fast. It is not "jumpy" so I don't believe it is a cable problem. Also, once when stopped, can't remember if in gear or not, it read 10 mph.
Diagnosis. A speedometer has a torsion spring which its main purpose is to hold the needle at zero, but it also puts some type of resistance to the needle when moving. My thoughts are that it is bad, whether it is dirty or broken or just weakened, there is a problem with it.
Question. I have a 100mph speedometer in the truck. I have several 85 mph spares. Can I use the torsion spring from one of the 85 to fix the 100 speedometer? If I just replace the 100 with an 85, does anything change? I already need to change the gear for the tires used, but does the gear need changed because of the different speedometer reading?
Thanks for you reading of my blah blah and any useful or useless replies.
Symptoms. It changes how much it is off. I have larger than original tires, so if I have read correctly, my speedometer should be reading slow. It doesn't, it reads fast and anywhere from 5 mph to 20 mph fast. It is not "jumpy" so I don't believe it is a cable problem. Also, once when stopped, can't remember if in gear or not, it read 10 mph.
Diagnosis. A speedometer has a torsion spring which its main purpose is to hold the needle at zero, but it also puts some type of resistance to the needle when moving. My thoughts are that it is bad, whether it is dirty or broken or just weakened, there is a problem with it.
Question. I have a 100mph speedometer in the truck. I have several 85 mph spares. Can I use the torsion spring from one of the 85 to fix the 100 speedometer? If I just replace the 100 with an 85, does anything change? I already need to change the gear for the tires used, but does the gear need changed because of the different speedometer reading?
Thanks for you reading of my blah blah and any useful or useless replies.
Hotshot
Joined: Apr 2009
Posts: 14,333
Likes: 247
From: Stanley, VA
Actually, it's a coil spring, not a torsion spring. The cable spins a magnet that is inside a metal cup. The spinning magnet "drags" the metal cup turning it against spring tension.
It sounds like your speedometer needs to be cleaned and lubed.
As for the 85/100 MPH speedometer. The speedometer themselves are interchangeable. The only difference is the location of the high beam jewel.
And you're correct, a larger diameter tire will make the speedometer read slow.
It sounds like your speedometer needs to be cleaned and lubed.
As for the 85/100 MPH speedometer. The speedometer themselves are interchangeable. The only difference is the location of the high beam jewel.
And you're correct, a larger diameter tire will make the speedometer read slow.
Thread Starter
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Elder User
Joined: Jan 2014
Posts: 898
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From: Chillicothe, IA
Thanks Mike, I will try cleaning it. Recommendations for cleaner and lube? I would think a teflon or silicone lube would be best for this. Carb or brake cleaner, or something gentler such as contact cleaner?
Hotshot
Joined: Apr 2009
Posts: 14,333
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From: Stanley, VA
Yeah, I would use something gentle. I don't know how the odometer dials and face of the speedo would react to strong cleaners. Then, like you said, Teflon or silicone spray.
Moderator & parts slinger
Joined: Feb 2007
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From: S/W Missouri, Polk county
I have seen a chart that shows the teeth count and colors...some where. And it might have a formula for the tire sizes?
https://www.ford-trucks.com/forums/1...ease-read.html
post #4
Ford Parts Specialist
Joined: Nov 2006
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From: Simi Valley, CA
Cars use the same nylon DRIVEN gears as F100/350, Bronco & Econoline (with one exception: 1978/80 Fiesta).
There's no Ford parts catalog info for a formula. What has to be known to get the correct DRIVEN gears, is the year/transmission type/rear axle ratio and tire size.
Lower pic: Types (see column left) that correspond to nylon DRIVEN gears shown in upper pic. TYPE #2 is for a Fiesta.
Upper/lower pics from 1973/79 Ford Passenger Car Parts catalog, the truck parts catalog doesn't list the colors.
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New User
Joined: Oct 2016
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Thanks the problem seems to be my autometer speedo. is not workingi use the drill on cable turns speedo. does nothing i will send it back thanks for info .
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Hotshot
Joined: Apr 2009
Posts: 14,333
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From: Stanley, VA
From the backside the speedometer drive spins counterclockwise, if you spin it clockwise, nothing will happen.
New User
Joined: Sep 2016
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So if the speed is reading linearly, at 5mph it is 6mph and at 10 mph it is 12mph, then more than likely it is just the gearing.
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