How do you know what speedometer cable transmission gear to use?

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nitronut

nitronut

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I put in a new speedometer cable in my 69 valiant today. 727 trans. (Third try, the last two times the speedometer needle jumped all over the place.) Today's installation was a success as the speedometer needle moves nice and smooth. My problem is, the speed reading is way off. Says I was doing 80 mph when I was traveling with cars I'm assuming were doing the 45mph limit. Went to a car show afterward and a couple folks said I need to change a gear where the speedometer cable connects to the transmission. Anyone know how to determine which is the right gear to correct this issue? Any info would be appreciated. Thanks...
 
There are charts including those in the FSM that outline which tooth gear is needed depending on your tire size and which rear end gears you have. Once you know this information we can determine which speedo gear you would need
 
nitronut said:
I put in a new speedometer cable in my 69 valiant today. 727 trans. (Third try, the last two times the speedometer needle jumped all over the place.) Today's installation was a success as the speedometer needle moves nice and smooth. My problem is, the speed reading is way off. Says I was doing 80 mph when I was traveling with cars I'm assuming were doing the 45mph limit. Went to a car show afterward and a couple folks said I need to change a gear where the speedometer cable connects to the transmission. Anyone know how to determine which is the right gear to correct this issue? Any info would be appreciated. Thanks...
Click to expand...

Hey Nitronut,

if the speedo cables you were using were new, you may not have had the gauge side inserted properly. The end of the cable is actually square and if it doesn't seat properly, it will cause the jumpy readings you were getting. When seated properly, you can feel the clip on the cable lock in place. (assuming this is clip style and not screw in type|).

Your other problem is probably caused by speedo gear in the transmission. Have you changed the rear end gear set or swapped out the rear end assembly? Is the transmission original to the car? You can determine the rear end ratio by checking for a ratio tag attached to the differential housing or remove the center section and check for ratio on ring gear. or, if an 8.25 remove the cover. This is most definitely the most accurate way to determine the ratio. Another less accurate way is to put the car on a lift , put the transmission in neutral. Put a mark on a tire and the drive shaft. turn the drive shaft and record how many revolutions the tire makes with each revolution of the drive shaft. If one turn of the driveshaft = 3.5 turns of the tire, your ratio is probably 3.55-1. Now, check here for the proper color speedo gear for your transmission.

Mopar Speedometer Gear Selection Guide - MyMopar.com

Also note, tire size is going to affect speedo readings as well. Hope this helps.


Cheers!!
 
If you have an non sure grip rear end, you will need to multiply by two if both tires are moving opposite directions. Took me awhile to figure that out. Works right if only one tire moves.
 
65 Dartman said:
There are charts including those in the FSM that outline which tooth gear is needed depending on your tire size and which rear end gears you have. Once you know this information we can determine which speedo gear you would need
Click to expand...
Thanks!
 
65 Dartman said:
There are charts including those in the FSM that outline which tooth gear is needed depending on your tire size and which rear end gears you have. Once you know this information we can determine which speedo gear you would need
Click to expand...
Thank You...
 
Mike69cuda said:
If you have an non sure grip rear end, you will need to multiply by two if both tires are moving opposite directions. Took me awhile to figure that out. Works right if only one tire moves.
Click to expand...
Good to know. Thanks..
 
65 Dartman said:
There are charts including those in the FSM that outline which tooth gear is needed depending on your tire size and which rear end gears you have. Once you know this information we can determine which speedo gear you would need
Click to expand...
Thank You!
 
AAndrews said:
Hey Nitronut,

if the speedo cables you were using were new, you may not have had the gauge side inserted properly. The end of the cable is actually square and if it doesn't seat properly, it will cause the jumpy readings you were getting. When seated properly, you can feel the clip on the cable lock in place. (assuming this is clip style and not screw in type|).

Your other problem is probably caused by speedo gear in the transmission. Have you changed the rear end gear set or swapped out the rear end assembly? Is the transmission original to the car? You can determine the rear end ratio by checking for a ratio tag attached to the differential housing or remove the center section and check for ratio on ring gear. or, if an 8.25 remove the cover. This is most definitely the most accurate way to determine the ratio. Another less accurate way is to put the car on a lift , put the transmission in neutral. Put a mark on a tire and the drive shaft. turn the drive shaft and record how many revolutions the tire makes with each revolution of the drive shaft. If one turn of the driveshaft = 3.5 turns of the tire, your ratio is probably 3.55-1. Now, check here for the proper color speedo gear for your transmission.

Mopar Speedometer Gear Selection Guide - MyMopar.com

Also note, tire size is going to affect speedo readings as well. Hope this helps.


Cheers!!
Click to expand...
Great info! Thank You very much...
 
as a guide with stock tires use your gear ratio.
i.e 3.23 gears would be a 31 or 32 tooth ...3.55 would be a 35 or 36 tooth speedometer gear.
 
Here ya go

Screenshot_20200510-113240_Samsung Internet.jpg
 
You need more teeth on the gear. If you have an extra one, stick it in there and see how much it changes. To get this right, you have to know exactly how far off it is now and how many teeth are on your present gear. And read the numbers on the speedo gear housing that is in the trans. You'll see there are numbers, and nubs to line up depending on how many teeth the gear has. Write down how far off it is now, what the teeth count is, what the teeth count of any gear you install, how far off that one is and on...The charts will get you close, but sometimes there isn't a gear available that will make it perfect. And sometimes they're expensive; depending where you shop.
 
CudaFactHackJob said:
You need more teeth on the gear. If you have an extra one, stick it in there and see how much it changes. To get this right, you have to know exactly how far off it is now and how many teeth are on your present gear. And read the numbers on the speedo gear housing that is in the trans. You'll see there are numbers, and nubs to line up depending on how many teeth the gear has. Write down how far off it is now, what the teeth count is, what the teeth count of any gear you install, how far off that one is and on...The charts will get you close, but sometimes there isn't a gear available that will make it perfect. And sometimes they're expensive; depending where you shop.
Click to expand...
Thanks a lot! Found out it takes a 40 tooth gear...
 
There is a speedometer app that you can get on your phone to check as you are driving, I've used it to check for correction after tire size change on my street cars.
 
Demonracer said:
There is a speedometer app that you can get on your phone to check as you are driving, I've used it to check for correction after tire size change on my street cars.
Click to expand...
Thank You! I'll give it a try after installing the new gear which has been ordered...
 
junk yard memory tool, divide teeth by 10 to get 'pretty close' rear ratio. 39 tooth is a 3.91, 27 tooth is a 2.76, 32 is a 3.23, etc. Works pretty good unless you have non stock tires. Easy way to find unknown rear in a roller with no jacks (SG or open) is to chalk the driveshaft (long diagonal mark across 1/2 of the shaft) and roll the car forward 1 tire revolution (use valve stem as reference) just count the mark as it rolls. Diagonal so you can pick up the estimated location if its on the other side. Just dont roll over yourself.
 
pishta said:
junk yard memory tool, divide teeth by 10 to get 'pretty close' rear ratio. 39 tooth is a 3.91, 27 tooth is a 2.76, 32 is a 3.23, etc. Works pretty good unless you have non stock tires. Easy way to find unknown rear in a roller with no jacks (SG or open) is to chalk the driveshaft (long diagonal mark across 1/2 of the shaft) and roll the car forward 1 tire revolution (use valve stem as reference) just count the mark as it rolls. Diagonal so you can pick up the estimated location if its on the other side. Just dont roll over yourself.
Click to expand...
Thanks!
 
You haven't found out it takes a 40 tooth gear until you drive it and it's accurate. Maybe it will be right. Maybe it will be a lot closer; and good enough.
 
CudaFactHackJob said:
You haven't found out it takes a 40 tooth gear until you drive it and it's accurate. Maybe it will be right. Maybe it will be a lot closer; and good enough.
Click to expand...
Thank you. I gave Mancini the tire size/rear end gear ratio and that's what they came up with (40 tooth). Hopefully it will be close but we'll see. All I have to do now is figure out how to install. Heard it's fairly easy.
 
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