rear end/speedometer question
10-25-2004, 04:20 PM
#1
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rear end/speedometer question
I have gotten my car all together, but I had to put a non origianl 3.36 posi rear-end in the car. I love the way it runs, but my speedometer is about 10 to 12 mph off. I am planning on sending the speedometer off to get the odometer fixed. Can the speedometer be recalibrated to the 3.36 rearend without the rearend being present. Forgive me if this is a dumb question. Thank you in advance.
10-25-2004, 04:51 PM
#2
Le Mans Master
Nothing wrong with the speedo as it is, the correction for rear gear is made by changing the speedo drive gear in the transmission. Not a difficult task, only one bolt to remove, unless you have to also change the gear on the tailshaft, but that's not likely. What was your old rear gear ratio?
10-25-2004, 06:13 PM
#4
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Don't have one in front of me, but in the AIM, it gives a listing of driven gears, with their colors, and the number of teeth.
When you pull your old one, check the color and #of teeth.
If you want the speedo to read faster, you'll need a gear with less teeth. Slower? More teeth.
If your speedo is reading 10% fast, and you want to slow it down, pick a gear with 10% fewer teeth, i.e. 22 tooth originally, 10% is roughly 2 teeth, pick a gear with 20 teeth, and so on.
The AIM should also tell you the "original" number of teeth in the driven gear for any particular rear end ratio. Chuck
When you pull your old one, check the color and #of teeth.
If you want the speedo to read faster, you'll need a gear with less teeth. Slower? More teeth.
If your speedo is reading 10% fast, and you want to slow it down, pick a gear with 10% fewer teeth, i.e. 22 tooth originally, 10% is roughly 2 teeth, pick a gear with 20 teeth, and so on.
The AIM should also tell you the "original" number of teeth in the driven gear for any particular rear end ratio. Chuck
10-25-2004, 06:39 PM
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The actual formula is 63360 X axle ratio divided by 3141.6 X tire diameter, then X by number of drive (not driven) teeth. For example, I have a '63 SW with a 3.70 rear end, tire diameter of 27" and an 8 tooth drive gear for a speedo gear of 22 teeth. My guess is that a 3.36 will put you in the 20 tooth range. The GM part numbers for speedo gears (unless they've changed them) are:
3987920 for a 20 tooth (blue)
3987921 for a 21 tooth (red)
3987922 fpr a 22 tooth (black)
3987920 for a 20 tooth (blue)
3987921 for a 21 tooth (red)
3987922 fpr a 22 tooth (black)
10-25-2004, 08:37 PM
#6
Le Mans Master
Originally Posted by bovette
I am not sure, how would I go about finding out? Thank you.
CA 3.36 Standard
CB 3.36 Positraction
CC 3.55 Positraction
CD 3.70 Positraction
CE 4.11 Positraction
CF 4.56 Positraction
CJ 3.08 Positraction
CX 3.70 Standard
CZ 3.08 Standard
You should be able to find one of these codes on your old housing, assuming you have it.
10-25-2004, 09:33 PM
#7
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DO NOT, DO NOT, DO NOT TOUCH YOUR SPEEDOMETER, YET!!!!!!!
As mentioned, most likely all that you need to do is to change the speedo DRIVEN gear on the side of the tranny. As mentioned, there are several different gears available. They range from 17 teeth to 25 teeth, and are color coded (but over the years, the color usually fades away and you end up with a neutral color gear). For a 3.36 rear, it is likely that you need a 20 or 21 tooth gear.
AFTER you change the gear in the tranny, go drive the car over a KNOWN measured mile (or 2mi, or 3mi, etc). DO NOT WORRY ABOUT THE SPEEDO READING--------------------CHECK THE ODOMETER READING!!!! That's what you want to get correct FIRST (change the tranny gear until the odometer is recording exactly in mile increments, if in a 5mi distance your odometer is within 1/2 of 1/10mi, that's close enough). And that can only be done by having the correct gear in the tranny. The ODOMETER is a DIRECT gear driven mechanism. The SPEEDOMETER is INDIRECTLY actuated by a rotating magnet inside of the speedo head. Chances are very good that if you get the correct speedo gear ratio in the tranny, so that the odometer reads accurately, then it is quite likely that the speedometer will also be correct (probably dead on!).
The odometer is NOT speed dependant-----------------it ONLY measures distance----------------thus, the only adjustment for the odometer is to have the correct gear ratio inside the tranny. Regardless of whether you drive 10mph or 90mph, the odometer is going to measure the distance that is traveled. And the speedometer is calibrated per what the rpm of the speedometer cable is turning. Does this make sense to you?
Then, after the odometer is reading accurately, if the speedo is STILL off, it will need to be removed and adjusted by a speedo shop (personally, if it were mine and it was only off by 2-3mph at 60-70mph, I'd leave it alone). To adjust the speedo so that the needle is reading correctly, the speedo head needs to be opened up for adjustment.
As mentioned, most likely all that you need to do is to change the speedo DRIVEN gear on the side of the tranny. As mentioned, there are several different gears available. They range from 17 teeth to 25 teeth, and are color coded (but over the years, the color usually fades away and you end up with a neutral color gear). For a 3.36 rear, it is likely that you need a 20 or 21 tooth gear.
AFTER you change the gear in the tranny, go drive the car over a KNOWN measured mile (or 2mi, or 3mi, etc). DO NOT WORRY ABOUT THE SPEEDO READING--------------------CHECK THE ODOMETER READING!!!! That's what you want to get correct FIRST (change the tranny gear until the odometer is recording exactly in mile increments, if in a 5mi distance your odometer is within 1/2 of 1/10mi, that's close enough). And that can only be done by having the correct gear in the tranny. The ODOMETER is a DIRECT gear driven mechanism. The SPEEDOMETER is INDIRECTLY actuated by a rotating magnet inside of the speedo head. Chances are very good that if you get the correct speedo gear ratio in the tranny, so that the odometer reads accurately, then it is quite likely that the speedometer will also be correct (probably dead on!).
The odometer is NOT speed dependant-----------------it ONLY measures distance----------------thus, the only adjustment for the odometer is to have the correct gear ratio inside the tranny. Regardless of whether you drive 10mph or 90mph, the odometer is going to measure the distance that is traveled. And the speedometer is calibrated per what the rpm of the speedometer cable is turning. Does this make sense to you?
Then, after the odometer is reading accurately, if the speedo is STILL off, it will need to be removed and adjusted by a speedo shop (personally, if it were mine and it was only off by 2-3mph at 60-70mph, I'd leave it alone). To adjust the speedo so that the needle is reading correctly, the speedo head needs to be opened up for adjustment.
Last edited by DZAUTO; 10-25-2004 at 09:37 PM.
10-26-2004, 12:26 PM
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Originally Posted by bovette
I have gotten my car all together, but I had to put a non origianl 3.36 posi rear-end in the car. I love the way it runs, but my speedometer is about 10 to 12 mph off. I am planning on sending the speedometer off to get the odometer fixed. Can the speedometer be recalibrated to the 3.36 rearend without the rearend being present. Forgive me if this is a dumb question. Thank you in advance.
10-26-2004, 11:55 PM
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Those gears are MUCH cheaper from Chev parts dept!
10-28-2004, 11:08 AM
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3987920 - blue 20-tooth driven gear for 3.36 - $4.52 list price, $3.57 jobber price; still available. 
10-28-2004, 11:15 AM
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Originally Posted by Stick
The actual formula is 63360 X axle ratio divided by 3141.6 X tire diameter, then X by number of drive (not driven) teeth. For example, I have a '63 SW with a 3.70 rear end, tire diameter of 27" and an 8 tooth drive gear for a speedo gear of 22 teeth. My guess is that a 3.36 will put you in the 20 tooth range. The GM part numbers for speedo gears (unless they've changed them) are:
3987920 for a 20 tooth (blue)
3987921 for a 21 tooth (red)
3987922 fpr a 22 tooth (black)
3987920 for a 20 tooth (blue)
3987921 for a 21 tooth (red)
3987922 fpr a 22 tooth (black)
The color black gear they say is 23 teeth (3.90??gear)
Yellow is 24 teeth for a 4.11 rear. Do you have the GM part #'s for yellow and green gears?
Thanks
10-29-2004, 12:35 AM
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OK, here is the WHOLE enchilada. But first, you need to know this.
There are TWO (2) different sizes of speedo DRIVE gears for the main shaft, big and little. The big gear has an OD of 1.84in, and the small one has an OD of 1.76in (these are for the 27 spline tranny tail shaft).
There are TWO (2) sets of DRIVEN gears, again, big and little.
The big gears range from 22 teeth to 25 teeth and are used with the little DRIVE gear.
The little gears range from 17 teeth to 22 teeth and are used with the big DRIVE gear (this last combo is the MOST common).
SMALL GEARS (to mate with the big drive gear)
3987917-17teeth-purple
3987918-18teeth-brown
3987919-19teeth-natural
3987920-20teeth-blue
3987921-21teeth-red
3987922-22teeth-gray
BIG GEARS (to made with the small drive gear)
3860345-22teeth-green
3860346-23teeth-black
3860347-24teeth-yellow
3860348-25teeth-orange
The last time I checked, all of the 17-25teeth gears were still available from Chev parts dept. As you can plainly see, there are TWO (2) different 22tooth gears.
There are TWO (2) different sizes of speedo DRIVE gears for the main shaft, big and little. The big gear has an OD of 1.84in, and the small one has an OD of 1.76in (these are for the 27 spline tranny tail shaft).
There are TWO (2) sets of DRIVEN gears, again, big and little.
The big gears range from 22 teeth to 25 teeth and are used with the little DRIVE gear.
The little gears range from 17 teeth to 22 teeth and are used with the big DRIVE gear (this last combo is the MOST common).
SMALL GEARS (to mate with the big drive gear)
3987917-17teeth-purple
3987918-18teeth-brown
3987919-19teeth-natural
3987920-20teeth-blue
3987921-21teeth-red
3987922-22teeth-gray
BIG GEARS (to made with the small drive gear)
3860345-22teeth-green
3860346-23teeth-black
3860347-24teeth-yellow
3860348-25teeth-orange
The last time I checked, all of the 17-25teeth gears were still available from Chev parts dept. As you can plainly see, there are TWO (2) different 22tooth gears.
Last edited by DZAUTO; 10-29-2004 at 12:44 AM.
10-29-2004, 08:57 AM
#17
Le Mans Master
Originally Posted by Ron Miller
Nothing wrong with the speedo as it is, the correction for rear gear is made by changing the speedo drive gear in the transmission. Not a difficult task, only one bolt to remove, unless you have to also change the gear on the tailshaft, but that's not likely. What was your old rear gear ratio?
