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Re: Changed speedo pinion, speedometer still off ?
[Re: MowP4rsn]
#118855
09/10/08 09:39 PM
09/10/08 09:39 PM
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Joined: Feb 2003
Posts: 4,204
Fort Worth, TX
Clair_Davis
master
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master
Joined: Feb 2003
Posts: 4,204
Fort Worth, TX
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If your speed is reading higher than you're actually going, you need to reduce the speed of the speedo cable... that is, make the pinion gear LARGER so it spins more slowly for a given driveshaft RPM. That means, add MORE teeth to the speedo pinion. The amount of extra teeth is a percentage, and you find it by dividing your speed by the real speed:
37mph/30mph = 1.233, so you need a little more than 23% MORE teeth on the next speedo pinion. If you've still got the 31-spline pinion in the trans, you need 1.233x31 = 38.23 teeth on the next one to bring the speedo speed down to reality.
Clair
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Re: Changed speedo pinion, speedometer still off ?
[Re: Clair_Davis]
#118856
09/10/08 11:39 PM
09/10/08 11:39 PM
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Joined: Mar 2005
Posts: 7,052
Madera, CA.
MowP4rsn
OP
master
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OP
master
Joined: Mar 2005
Posts: 7,052
Madera, CA.
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Clair, OK, I can understand it they way you put it, but I used the excel program in the tech archives, and this is what it came up with, hence my post 
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Re: Changed speedo pinion, speedometer still off ?
[Re: MowP4rsn]
#118858
09/10/08 11:54 PM
09/10/08 11:54 PM
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Joined: Nov 2003
Posts: 36,041
Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,041
Lincoln Nebraska
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put a piece of flat stock on the top of one of your rear tires & (1) get your height & (2) you know your rear gear ratio. go into the archves & find the correct pinion you need from the MP chart. Made my odometer spot on. the speedometer can still be off if the magnetic gismo inside the speedometer has lost it's calibration even after you select the correct speedo pinion. The adapter on the side of the trans just rotate it to correspond to the # of teeth pinion you end up needing & you're good.
live every 24 hour block of time like it's your last day on earth
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Re: Changed speedo pinion, speedometer still off ?
[Re: MowP4rsn]
#118860
09/11/08 12:14 AM
09/11/08 12:14 AM
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Joined: Nov 2003
Posts: 36,041
Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,041
Lincoln Nebraska
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Jack up the rear & double check your actual gear ratio, measure the actual height of your rear tires as different brands of the same size have different heights. Get the right speedo pinion in there via the simplified MP chart then pull over that motorcycle cop again & check your speedometer needle calibration & if it's off contact a specialized shop that recalibrates them.
live every 24 hour block of time like it's your last day on earth
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Re: Changed speedo pinion, speedometer still off ?
[Re: MowP4rsn]
#118862
09/11/08 01:21 AM
09/11/08 01:21 AM
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Joined: Nov 2003
Posts: 36,041
Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,041
Lincoln Nebraska
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just start burning some rubber you'll find him.
live every 24 hour block of time like it's your last day on earth
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Re: Changed speedo pinion, speedometer still off ?
[Re: Clair_Davis]
#118863
09/11/08 08:04 AM
09/11/08 08:04 AM
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Anonymous
Unregistered
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Anonymous
Unregistered
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Quote:
If your speed is reading higher than you're actually going, you need to reduce the speed of the speedo cable... that is, make the pinion gear LARGER so it spins more slowly for a given driveshaft RPM. That means, add MORE teeth to the speedo pinion. The amount of extra teeth is a percentage, and you find it by dividing your speed by the real speed:
37mph/30mph = 1.233, so you need a little more than 23% MORE teeth on the next speedo pinion. If you've still got the 31-spline pinion in the trans, you need 1.233x31 = 38.23 teeth on the next one to bring the speedo speed down to reality.
Clair
 tire diameter may not be as advertised once installed and loaded down.
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Re: Changed speedo pinion, speedometer still off ?
#118864
09/11/08 08:54 AM
09/11/08 08:54 AM
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Joined: Apr 2005
Posts: 4,922
new berlin wisconsin
Mr T2U
master
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master
Joined: Apr 2005
Posts: 4,922
new berlin wisconsin
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one way to check your speedo for accuracy is to drive past mile markers on the side of the freeway and compare it to your miles turned on the speedo.
perception is 90% of reality
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Re: Changed speedo pinion, speedometer still off ?
[Re: Mr T2U]
#118865
09/11/08 09:47 AM
09/11/08 09:47 AM
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Joined: Jan 2003
Posts: 18,424
UPPER MICHIGAN, MARQUETTE COUN...
NITROUSN
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 18,424
UPPER MICHIGAN, MARQUETTE COUN...
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Quote:
one way to check your speedo for accuracy is to drive past mile markers on the side of the freeway and compare it to your miles turned on the speedo.
That will work for the odometer part but not the speed part.
To many varibles in the speedo department. Install a gear that works for your combo. Sounds to me like you should of used a 32 or 33 tooth to dial it in.
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Re: Changed speedo pinion, speedometer still off ?
[Re: NITROUSN]
#118866
09/11/08 10:34 AM
09/11/08 10:34 AM
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Joined: Nov 2003
Posts: 36,041
Lincoln Nebraska
RapidRobert
Circle Track
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Circle Track
Joined: Nov 2003
Posts: 36,041
Lincoln Nebraska
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If he goes a measured mile on the interstate mile markers in exactly 1 minute @60 MPH then his speedometer part is good but only if being accurate at a given speed(60 MPH in this case) automatically means that the speedo needle is accurate throughout its range. A speedometer shop could answer that Q in 1 minute over the phone but 1st the correct speedo pinion needs to be in there.
live every 24 hour block of time like it's your last day on earth
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Re: Changed speedo pinion, speedometer still off ?
[Re: Dart 340]
#118868
09/11/08 05:37 PM
09/11/08 05:37 PM
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Joined: Jan 2003
Posts: 25,858
Rio Linda, CA
John_Kunkel
Too Many Posts
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Too Many Posts
Joined: Jan 2003
Posts: 25,858
Rio Linda, CA
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You might also find that if you choose the pinion that makes the speedo correct but the odometer incorrect, the speedo won't be correct throughout its range; IOW, it might be dead on at 60 mph but way off at a higher or lower speed.
It's best to choose a pinion that makes the odometer correct and measure the speedo error (stopwatch/mile markers) at several speeds and write them down (or memorize).
The INTERNET, the MISinformation superhighway
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Re: Changed speedo pinion, speedometer still off ?
[Re: John_Kunkel]
#118869
09/11/08 05:54 PM
09/11/08 05:54 PM
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Joined: Nov 2007
Posts: 21,831
Kirkland, Washington
Pacnorthcuda
Too Many Posts
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Too Many Posts
Joined: Nov 2007
Posts: 21,831
Kirkland, Washington
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Quote:
You might also find that if you choose the pinion that makes the speedo correct but the odometer incorrect, the speedo won't be correct throughout its range; IOW, it might be dead on at 60 mph but way off at a higher or lower speed.
It's best to choose a pinion that makes the odometer correct and measure the speedo error (stopwatch/mile markers) at several speeds and write them down (or memorize).
Or find speed display units out in public (they're pretty common in the Northwest) and compare.
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Re: Changed speedo pinion, speedometer still off ?
[Re: Pacnorthcuda]
#118871
09/12/08 11:53 PM
09/12/08 11:53 PM
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Joined: Mar 2005
Posts: 7,052
Madera, CA.
MowP4rsn
OP
master
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OP
master
Joined: Mar 2005
Posts: 7,052
Madera, CA.
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Quote:
Quote:
You might also find that if you choose the pinion that makes the speedo correct but the odometer incorrect, the speedo won't be correct throughout its range; IOW, it might be dead on at 60 mph but way off at a higher or lower speed.
It's best to choose a pinion that makes the odometer correct and measure the speedo error (stopwatch/mile markers) at several speeds and write them down (or memorize).
Or find speed display units out in public (they're pretty common in the Northwest) and compare.
I did 2 passes by units on a street not far from where I live, and it appears my speedo is off 6 to 9 mph, or thereabouts.
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Re: Changed speedo pinion, speedometer still off ?
[Re: MowP4rsn]
#118873
09/13/08 09:47 AM
09/13/08 09:47 AM
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Joined: Aug 2007
Posts: 2,356
Kentucky
dfsmopars
top fuel
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top fuel
Joined: Aug 2007
Posts: 2,356
Kentucky
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I had my speedo calibrated by Auto Instruments, bought way taller tires and changed the gear ratio. I went by the chart in archives and it put me right on. I have 28.5" tires and 3.55 gears so I bought a 33 tooth pinion. Good luck!
‘72 Charger, 5.9 Magnum, Tremec 5 spd., Pro-Touring
‘14 Big Horn, Quad Cab, 4x4, 1500
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It might be the magnet then...considering it IS over 40 years old. I did notice the speedo needle didn't JUMP as far or as fast after the pinion gear change.
