Speedometer Calibration Help Needed

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dustermaniac

dustermaniac

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Does anyone out here know how I can recalibrate my speedometer? It was reading properly until I changed out my rear tires and now it is 13 -15 MPH off from what speed I am traveling at. Example: If I am traveling at 65 MPH, my Speedometer is reading 50 MPH. I have now got my 5th speeding ticket as a result of improper readings from my Speedometer and I am using my GPS as a temporary Speedometer. I am not sure If I would need to count the gear and reinstall the gear back into the transmission in another position from where I originally set it. Any assistance is greatly appreaciated as I cannot afford another ticket, I do not want to lose my license.
 
We have a transmission shop in town that does it, you might check there.
 
You need a different speedometer pinion gear. To make it read more you need a gear with less teeth. I have a chart of what speedo gear to use with what size tires and what gear ratio you have but I can't figure out how to post it so if you can tell me the tire size and the gear ratio I can tell you what speedo pinion gear to use.

BTW: The speedometer pinion gear is installed in the tailshaft of the transmission. It's real easy to change out. I just did mine the other day and it took longer to jack up the car and put it on stands than it did to change the gear.
 
@fishy68 and dustermaniac hope you don´t mind when i jump on the train.

got a 3,23:1 ratio and 235/60/15 tire = 26.1" if i have it figured out the correct way it should be a 20teeth pinon for me, or? (btw. old pushbutton tranny)
 
lilredridinghoo said:
@fishy68 and dustermaniac hope you don´t mind when i jump on the train.

got a 3,23:1 ratio and 235/60/15 tire = 26.1" if i have it figured out the correct way it should be a 20teeth pinon for me, or? (btw. old pushbutton tranny)
Click to expand...

I don't know for sure if the pushbutton trans. are the same setup as the newer trans. but I think they are. If that's the case you need a 33 tooth gear.

Longgone said:
This chart will give you an idea of the correct speedometer gear you`ll need with your tire size. http://www.mymopar.com/speedometer.htm
Click to expand...

I forgot about that one. Good job LG.
 
I`ve noticed with a standard tire diameter (around 26.5") the speedometer gear tooth count closely mimics the rear end gears. For example a 3.23:1 ring and pinion gear usually takes around a 32 tooth speedo gear, a 3.55 = 35 tooth, 3:73 = 37, 4.10 = 41, and so on. It`s always very close but slight differences in tire height will change things by a tooth or two. There`s another formula that`s much more accurate which uses actual tire height , sidewall ratio, and rim size to formulate the exact gear. I`ll see if I can find it and post it.
 
For Timo with the push button trans:

For your car I believe you are correct that it needs a 20 tooth gear. They changed the design of the speedometer pinon in 1966. Most of the charts online are for the newer gears (which have more teeth) Unfortunately the older style gears are much harder to find.
 
duster maniac, You might be able to correct it buy installing a smaller
driven gear. You're speedo is approx. 24% slow. For example if you have
a 36 tooth driven gear you would need a 28 tooth driven gear. I don't know
if they make such a small gear. If not, then as Fishy68 says you will need to
change the speedo pinion gear. Remove the driven gear(the one the speedo
cable is attached to) and count, then do the math.
 
I ended up going with a Auto Meter electronic speedo for my truck. Push the button, drive a measured 2 miles and presto, it's calibrated. If I make any changes, all I have to do is repeat the process. Granted, it was more expensive than a mechanical speedo but by the time I would have had a local shop make me a custom gearbox for the speedo, the electronic was cheaper in the long run.

Of course if you want to keep the interior stock you really have no other option than change the gear or get a custom adapter made.
 
D Cluley said:
For Timo with the push button trans:

For your car I believe you are correct that it needs a 20 tooth gear. They changed the design of the speedometer pinon in 1966. Most of the charts online are for the newer gears (which have more teeth) Unfortunately the older style gears are much harder to find.
Click to expand...


that´s exactly the pbm.... i´ve got an offer from tfpatty, but round about 120 dollars for a little plastic pinion is way far to expensive
 
This is exactly what I needed to recalibrate the speedo in 'Cuda.

Thanks for posting.
 
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