Speed off in used Mustang, display

Tiny
LEFTNEWKNEE
  • MEMBER
  • 1968 FORD MUSTANG
  • 6 CYL
  • 2WD
  • MANUAL
  • 88,000 MILES
I own a 1968 ford mustag 6 cylinder 200ci. St. 6.
the problem that I am having is the speedometer is not correct, the needle goes down pat the {0} mph when I am stop and when I drive the car lets say 10 mph or 15 mph the guage shows {0] mph,
when I drive past a posted rader sign my mph is off at least 15 mph,
from what is posted on the sign, can you tell me what I should do? And what I might try to correct this problem.
i bought this car as used, I had one before I went to viet nam, and gave my brother the first one and I was luckey to find another one,
leftnewknee@aol. Com
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Thursday, January 6th, 2011 AT 3:00 AM

9 Replies

Tiny
FORD MAN
  • MEMBER
Are you running STOCK size wheels & tires? That car came from the factory with 14-inch wheels ! If you have stock wheels, then the speedo gear in the trans is junk & needs to be replaced. Ask around (your friends) about a good trans shop, or all else fails. The Ford dealer in your area.

Ford Man in Michigan
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Thursday, January 6th, 2011 AT 3:20 AM
Tiny
CARADIODOC
  • EXPERT
Tire size would be irrelevant when standing still. Sounds like the pointer is turned on the speedometer shaft. You can try holding the shaft with a needle nose pliers, then rotate the pointer a little. There is normally a mechanical stop that the spinning metal disc hits to set the position when stopped and to keep the pointer from going the wrong way when you're backing up. If you carefully turn the pointer clockwise past the maximum reading, that stop will be reached again, it will prevent the disc from turning further, then you can force the pointer a little more so it rotates on the shaft. Do that either or both ways until it reads "0" when you're standing still, then carefully press the pointer onto the shaft a little tighter. If it spins too freely, you can glue it in place with a small dot of fingernail polish on the backside of the pointer. You can also glue it with a very small dab of silicone gasket sealer applied with a toothpick, but if you ever have to move the pointer again, it will be hard to get that stuff off. Fingernail polish will crack off easily, then you can reapply it later.

Caradiodoc
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Thursday, January 6th, 2011 AT 12:59 PM
Tiny
LEFTNEWKNEE
  • MEMBER
Thanks caradiodoc for the answer, that seems to be the answer, thanks again.
James
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Thursday, January 6th, 2011 AT 4:22 PM
Tiny
CARADIODOC
  • EXPERT
You're welcome.

I don't mean to dismiss Ford Man's reply. First a little "theory". When the pointer is always off by 15 mph no matter how fast you're going, the pointer just needs to be repositioned. The clue is yours is reading below 0 mph when standing still. It can't do that because of the mechanical stop, so there's proof it's the pointer.

When the wrong speedometer gear is in the transmission, or you change tire size, your speedometer will always be off by a percentage, not a specific amount. That means it will read 0 mph when you're standing still, 15 mph too fast at 50 mph, (in this example), and 30 mph too fast at 100 mph. The faster you go, the greater the error. That's not what's happening in your car.

That said, a former coworker had a Grand Caravan that always read about 7 mph higher than it should have. It didn't matter if he went 30 mph or 60 mph. It always read 7 mph too much. But it did read 0 mph when standing still. This was a different kind of speedometer than yours though. Instead of a speedometer cable, it used a speed sensor and a computer to run a "stepper" motor which has magnetic coils that pulse the pointer to the desired positions. It has a mechanical stop for the pointer itself, not the armature of the motor. According my story the pointer needed to be repositioned, but the coworker replaced the gear in the transmission and the speed was correct at every speed after that. I wanted to argue with him but you can't argue with success. Later I learned the reason for his success. The scale of the speedometer wasn't linear, meaning the distance from 0 - 30 mph was different than from 30 - 60 mph. So while the percentage of error remained constant, the non-linear scale corrected most of the error. That can't happen to a cable-driven speedometer.

Caradiodoc
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Friday, January 7th, 2011 AT 5:15 AM
Tiny
FORD MAN
  • MEMBER
Caradiodoc. Yep, your dissing me. No big deal ! Being old enuff to have driven that year of car brand new.80% of the time a when a speedo was off (by however much), tire size had been increased. If my menory serves me right.(1-'61 Comet, 3-'62 Galixies, several years ('62, '64, '65, '69) Thunderbirds), ALL of those speedos turned the correct way (speed inducated) while going backward ! The little nylon gear in the tranny only turned 1 direction, it didn't give a damn which gear the car was in. The only way the "pointer" could have been moved off the correct position, is that somebody ran the speedo cable BACKWARD trying to remove miles off the odometer !

Leftnewknee. Good Luck with your problem, & I hope everything here (2CarPros) helps to fix said problem !

Ford Man in Michigan
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Friday, January 7th, 2011 AT 6:00 PM
Tiny
CARADIODOC
  • EXPERT
How did they make the gear turn only one way? Did something lift it up when in reverse or was there some type of over-running clutch? Seems like either of those would complicate it unnecessarily.

I never changed a gear in a Ford but I used to do a lot of them in my old Chryslers.

Caradiodoc
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Friday, January 7th, 2011 AT 6:20 PM
Tiny
FORD MAN
  • MEMBER
Speedo gear was on it's own shaft ! If you remember. Back in the day, we could "push start" an automatic tranny? Your old 727's could run the speedo while going backward, check out the push botton 727 trans off a '57 Chrysler with the 392 hemi motor.

I could shoot holes in your suggestion on the speed inducator repair
(have you ever taken 1 of those apart), but it's not worth the effort. I just hope Leftnewknee looks at how that part is made "before" he tries the repair you suggested.
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Friday, January 7th, 2011 AT 6:45 PM
Tiny
CARADIODOC
  • EXPERT
Yup. All my experience was with the older Chrysler stuff. I have Challengers and Volares. On those it was real easy to pop the pointers off. I really shouldn't suggest grabbing the shaft with a needle nose pliers. They're too easy to bend if you're not careful.

Caradiodoc
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Friday, January 7th, 2011 AT 7:01 PM
Tiny
FORD MAN
  • MEMBER
The little pointer & assembly are all made with alum, & bend so freakin easy. The shaft (speedo cable) isn't in the speedo if the cluster is out of the dashboard. The only way to do this repair is to pull the cluster out. His best way to fix the problem (pointer is broken off it's assembly, & has been glued back on) is to buy another speedo cluster. "IF" that is the problem ! He may also have a worn out "worm" (the speedo cable), either stretched or nearly broken. Being as that part never got the maintaince (pulled apart & lubed) that said part required.

I'll sign this. Motor City Boy. Forever.
Ford Man in Michigan
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Friday, January 7th, 2011 AT 7:49 PM

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