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Changed speedo pinion, speedometer still off ?

Posted By: MowP4rsn

Changed speedo pinion, speedometer still off ? - 09/11/08 01:13 AM

I just had the original 31 tooth (green) speedo pinion changed out for a 30 tooth (yellow) speedo pinion since I've gone to 235/75-15 tires, but the speedometer reading is still off a little (I asked a motorcycle cop to pace me when I was waiting for a light, and when he called out 30mph my speedo read 37-ish).

I the shop that installed it for me that the retainer indexing be set with the middle mark line up with the mark on the housing, and I wonder if changing it will help with that or not.

Am I going to have to find a 29 tooth speedo pinion gear?
Posted By: Clair_Davis

Re: Changed speedo pinion, speedometer still off ? - 09/11/08 01:39 AM

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
Posted By: MowP4rsn

Re: Changed speedo pinion, speedometer still off ? - 09/11/08 03:39 AM

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

Attached picture 4678479-Speedo-calc.JPG
Posted By: MowP4rsn

Re: Changed speedo pinion, speedometer still off ? - 09/11/08 03:42 AM

And that was also the reason why I wondered if the indexing of the retainer might change the speedometer reading

Then again, I am assuming the gear ratio in my rear axle hasn't been changed from the 3.23 according to the fender tag
Posted By: RapidRobert

Re: Changed speedo pinion, speedometer still off ? - 09/11/08 03:54 AM

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.
Posted By: MowP4rsn

Re: Changed speedo pinion, speedometer still off ? - 09/11/08 04:03 AM

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.

Thanks for another perspective!
Posted By: RapidRobert

Re: Changed speedo pinion, speedometer still off ? - 09/11/08 04:14 AM

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.
Posted By: MowP4rsn

Re: Changed speedo pinion, speedometer still off ? - 09/11/08 04:59 AM

Something else to add to the "To Do list"...I might have to "borrow" my brother's GPS as a back-up speedo until I can get the current situation sorted out.

Robert, I don't think I'll have the same good fortune of finding a motorcycle cop to pace me, but who knows? Anyway, thanks for the ideas!
Posted By: RapidRobert

Re: Changed speedo pinion, speedometer still off ? - 09/11/08 05:21 AM

just start burning some rubber you'll find him.
Posted By: Anonymous

Re: Changed speedo pinion, speedometer still off ? - 09/11/08 12:04 PM

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.
Posted By: Mr T2U

Re: Changed speedo pinion, speedometer still off ? - 09/11/08 12:54 PM

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.
Posted By: NITROUSN

Re: Changed speedo pinion, speedometer still off ? - 09/11/08 01:47 PM

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.
Posted By: RapidRobert

Re: Changed speedo pinion, speedometer still off ? - 09/11/08 02:34 PM

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.
Posted By: Dart 340

Re: Changed speedo pinion, speedometer still off ? - 09/11/08 03:31 PM

I had the same problem. My odo would mark out a mile exactly with the correct pinion but was 20 mph off. I adjusted the ratio on the pinion after a few trys and the speedo is accurate now but the odo is off. What this means is the speedo internals that move the needle need to be calibrated. Will be pulling over the winter to get it all right.
Posted By: John_Kunkel

Re: Changed speedo pinion, speedometer still off ? - 09/11/08 09:37 PM


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).
Posted By: Pacnorthcuda

Re: Changed speedo pinion, speedometer still off ? - 09/11/08 09:54 PM

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.
Posted By: MowP4rsn

Re: Changed speedo pinion, speedometer still off ? - 09/12/08 05:11 AM

I know of 2 nearby, since they are at either side of a street that has a school on it, so maybe tomorrow I can run by there.

Posted By: MowP4rsn

Re: Changed speedo pinion, speedometer still off ? - 09/13/08 03:53 AM

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.
Posted By: 79powerwagon

Re: Changed speedo pinion, speedometer still off ? - 09/13/08 11:12 AM

Or do what I did- throw away all of the factory gauges and buy high quality new ones!
Posted By: dfsmopars

Re: Changed speedo pinion, speedometer still off ? - 09/13/08 01:47 PM

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!
Posted By: Pacnorthcuda

Re: Changed speedo pinion, speedometer still off ? - 09/13/08 02:27 PM

When doing this check I like to have a passenger, tell the passenger to just continuously state the speed indicated on the radar--like 43,43,43,43,44,43,43 That way the driver doesn't even have to look at it and can instead focus on the car's speedo (and where he's driving!), you'll find you come away with a much better idea of the difference.
Posted By: RapidRobert

Re: Changed speedo pinion, speedometer still off ? - 09/13/08 02:36 PM

Quote:

When doing this check I like to have a passenger, tell the passenger to just continuously state the speed indicated on the radar--like 43,43,43,43,44,43,43 That way the driver doesn't even have to look at it and can instead focus on the car's speedo (and where he's driving!), you'll find you come away with a much better idea of the difference.

YES 2 people needed to get it perfect,driver to keep the speedo exactly on your chosen speed & one to eyeball the rapidly changing sign especially as the one here is on a downhill ramp .
Posted By: 5thAve

Re: Changed speedo pinion, speedometer still off ? - 09/13/08 04:06 PM

Quote:

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.



The speedometer usually works off the same parts that the odometer does so if that is working right then he will know he has the right gear and theres something wrong with the speedometer.
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