speedo/tranny driven gear help!!
Thread Starter
Drifting
Joined: Sep 2004
Posts: 1,532
Likes: 4
From: Fairview Heights Illinois, near Saint Louis MO, STL C3 Shark
speedo/tranny driven gear help!!
My 4 speed 76 vette has a green 22 tooth driven gear. The speedometer is off, and I've done calculations and have found that I need a 19 tooth to even things out. I ordered a 19 tooth natural color driven gear from Chevrolet that came in today.
The green (original) driven gear is approx 1 7/8" diameter across the gear teeth. THe new one, is only 1 3/4" in diameter. I installed it not noticing and went for a test drive, got acouple of hundred feet from the house and noticed the speedo wasn't working at all.
Took it out, put it back in, figured I just didn't seat it right. No difference. PUlled it out, inpsected it, no stripped teeth, compared to the green on and noted the measurement difference above.
So, my problem - none of the online sites I find say that there even was a green, 22 tooth, 1 7/8" driven gear. Only a grey one.
In any event, where can I get the right 1 7/8" 19 tooth driven gear???
The green (original) driven gear is approx 1 7/8" diameter across the gear teeth. THe new one, is only 1 3/4" in diameter. I installed it not noticing and went for a test drive, got acouple of hundred feet from the house and noticed the speedo wasn't working at all.
Took it out, put it back in, figured I just didn't seat it right. No difference. PUlled it out, inpsected it, no stripped teeth, compared to the green on and noted the measurement difference above.
So, my problem - none of the online sites I find say that there even was a green, 22 tooth, 1 7/8" driven gear. Only a grey one.
In any event, where can I get the right 1 7/8" 19 tooth driven gear???
Thread Starter
Drifting
Joined: Sep 2004
Posts: 1,532
Likes: 4
From: Fairview Heights Illinois, near Saint Louis MO, STL C3 Shark
Did I move enough teeth to need a different drive gear??? need help, thought this was going to be an easy 15 minute job.
From the following website:
http://www.tciauto.com/components/speedometer_gears.htm
my green 22 tooth doesn't exist.......
From the following website:
http://www.tciauto.com/components/speedometer_gears.htm
my green 22 tooth doesn't exist.......
Thread Starter
Drifting
Joined: Sep 2004
Posts: 1,532
Likes: 4
From: Fairview Heights Illinois, near Saint Louis MO, STL C3 Shark
Greg, I don't know what the rear is, I can only guess it is 3.55 or 3.73 based off of the rpms at/near 70 mph = 33-3500 rpms
The speedometer reads slow by 10-12mph with the 22 tooth gear in place. 19 teeth reduction should increase the speedometer speed about 12 mph (I came to that conclusion at some point)
The speedometer reads slow by 10-12mph with the 22 tooth gear in place. 19 teeth reduction should increase the speedometer speed about 12 mph (I came to that conclusion at some point)
Thread Starter
Drifting
Joined: Sep 2004
Posts: 1,532
Likes: 4
From: Fairview Heights Illinois, near Saint Louis MO, STL C3 Shark
Hmm.. The tranny is not original, if that makes a difference. How fast is way fast?
Here is my logic
19teeth/22 teeth = 86 % reduction in speedometer gear (increased speed on speedometer
60mph/70mph = 85 %, so going from 22 teeth to 19teeth, should increase my speedometer speed 86 percent, or from 60 to 70 mph (69.47 actual mph)
But my confusion is the non-existant 22 tooth green driven gear that I just put back in. I can't really tell the color/material of the drive gear.
Here is my logic
19teeth/22 teeth = 86 % reduction in speedometer gear (increased speed on speedometer
60mph/70mph = 85 %, so going from 22 teeth to 19teeth, should increase my speedometer speed 86 percent, or from 60 to 70 mph (69.47 actual mph)
But my confusion is the non-existant 22 tooth green driven gear that I just put back in. I can't really tell the color/material of the drive gear.
Just another Corvette guy
Joined: Feb 1999
Posts: 8,518
Likes: 3,865
From: Palm Springs, CA.
I should have metered the discrepancy before I pulled my trans. When travelling 70mph I was reading approx. 90mph. I have the orig. trans and diff. Previous owner cured his bouncing speedo by installing the new driven gear. He just wasn't that fussy about getting the correct one. 19 teeth may be right for your application from the algebra you've "ciphered". I know this can be a bitch to get right sometimes.
Thread Starter
Drifting
Joined: Sep 2004
Posts: 1,532
Likes: 4
From: Fairview Heights Illinois, near Saint Louis MO, STL C3 Shark
greg, thanks for responding.
Anyone else w/ any information? did I move too far down the driven gear road and now need a new drive gear?
The diameter of the two driven gears is the reason the new one (smaller diameter) won't contact the drive gear teeth.
Jeremy
Anyone else w/ any information? did I move too far down the driven gear road and now need a new drive gear?
The diameter of the two driven gears is the reason the new one (smaller diameter) won't contact the drive gear teeth.
Jeremy
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Racer
Joined: Apr 2004
Posts: 401
Likes: 0
From: IL
Few years back I was installing a 200R4 in my Olds convertible and started looking for speedo info. Been collecting GM info as a result, your green 22 teeth driven = GM#3860345, though it's likely that this number has been superseded many times by now.
Let's start from the basics, tire size(P255-?), gear ratio, drive gear count, and driven gear count to come up with a combination for your setup. I think you may just be looking at the wrong transmission type when picking your gear number.
Later.
Let's start from the basics, tire size(P255-?), gear ratio, drive gear count, and driven gear count to come up with a combination for your setup. I think you may just be looking at the wrong transmission type when picking your gear number.
Later.
Melting Slicks
Joined: Feb 2001
Posts: 3,330
Likes: 13
From: Europe , Luxembourg
Hi
OK, I have the same problem. Had a clear driven gear in my M21 close ratio with 3.70 rear end.
When I installed the GM T56 Viper 6 speed tranny, I got told that I can use the original M21 gear driven setup and thats what I did.
The clear gear was hard to insert and stripped threads within a few hundret miles.
I installed a red one with more teeth and it's holding fine, but reads wrong.
The red gear outside diameter is much less than the clear one was.
I guess, you can not just install different gears without also changing the drive gear inside the tranny.
I will have to contact D&D to ask what they recommend as the T56 is from them.
Funny enough, all vendors carry all the different driven gears, but say nowhere what drive gear is required.
Günther
OK, I have the same problem. Had a clear driven gear in my M21 close ratio with 3.70 rear end.
When I installed the GM T56 Viper 6 speed tranny, I got told that I can use the original M21 gear driven setup and thats what I did.
The clear gear was hard to insert and stripped threads within a few hundret miles.
I installed a red one with more teeth and it's holding fine, but reads wrong.
The red gear outside diameter is much less than the clear one was.
I guess, you can not just install different gears without also changing the drive gear inside the tranny.
I will have to contact D&D to ask what they recommend as the T56 is from them.
Funny enough, all vendors carry all the different driven gears, but say nowhere what drive gear is required.
Günther
Just another Corvette guy
Joined: Feb 1999
Posts: 8,518
Likes: 3,865
From: Palm Springs, CA.
Jeremy,
I think my problem is solved. I just spoke with my guy at the local Chev. dealer. He went to the parts book and apparently is has a chart in the back to use for application. My 3.55:1 rear with M20 4 speed calls for a blue, 20 tooth driven gear, part number 3987920 and he has it in stock. Drive gear should remain the same. This sounds correct, I'll let you know after re-assembly. If you'd like to try my 18 tooth driven gear in yours I'd be happy to send it to you.
Greg
I think my problem is solved. I just spoke with my guy at the local Chev. dealer. He went to the parts book and apparently is has a chart in the back to use for application. My 3.55:1 rear with M20 4 speed calls for a blue, 20 tooth driven gear, part number 3987920 and he has it in stock. Drive gear should remain the same. This sounds correct, I'll let you know after re-assembly. If you'd like to try my 18 tooth driven gear in yours I'd be happy to send it to you.
Greg
Thread Starter
Drifting
Joined: Sep 2004
Posts: 1,532
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From: Fairview Heights Illinois, near Saint Louis MO, STL C3 Shark
Originally Posted by Greg
Jeremy,
I think my problem is solved. I just spoke with my guy at the local Chev. dealer. He went to the parts book and apparently is has a chart in the back to use for application. My 3.55:1 rear with M20 4 speed calls for a blue, 20 tooth driven gear, part number 3987920 and he has it in stock. Drive gear should remain the same. This sounds correct, I'll let you know after re-assembly. If you'd like to try my 18 tooth driven gear in yours I'd be happy to send it to you.
Greg
I think my problem is solved. I just spoke with my guy at the local Chev. dealer. He went to the parts book and apparently is has a chart in the back to use for application. My 3.55:1 rear with M20 4 speed calls for a blue, 20 tooth driven gear, part number 3987920 and he has it in stock. Drive gear should remain the same. This sounds correct, I'll let you know after re-assembly. If you'd like to try my 18 tooth driven gear in yours I'd be happy to send it to you.
Greg
My tires are 255 60 15's, rear end? no clue probably 3.55 or 3.73, drive gear, can't tell what color/material it is, can barely see it through the very small opening in the side of the tailshaft.
As mentioned by WESCH, Why doesn't anybody know what drive gear is required given the driven gear??? I'll call the chevy parts department in a few minutes
Just another Corvette guy
Joined: Feb 1999
Posts: 8,518
Likes: 3,865
From: Palm Springs, CA.
Jeremy is your car an automatic? My 4 speed gear is way smaller than your measurements. Approx. 7/8" diameter and approx. 1 & 3/4" circumfrence. If you do have a 4 speed and would like to try this one, just say the word.
Greg
Greg
Thread Starter
Drifting
Joined: Sep 2004
Posts: 1,532
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From: Fairview Heights Illinois, near Saint Louis MO, STL C3 Shark
Originally Posted by Greg
Jeremy is your car an automatic? My 4 speed gear is way smaller than your measurements. Approx. 7/8" diameter and approx. 1 & 3/4" circumfrence. If you do have a 4 speed and would like to try this one, just say the word.
Greg
Greg
DOH!!!!!!!!!!!!!!!!
your right, I meant 3/4" vs 7/8" diameter, i got too finger happy when typing and added a 1. :o
No, mines a 4 speed. I used a caliper to measure across the teeth of the driven gear. The one that works (but wrong speed on speedo) is 7/8" across the teeth. The one that doesn't is 3/4". Sounds like your's is like the one I just bought. Thanks for the offer though.
Jeremy
Racer
Joined: Apr 2004
Posts: 401
Likes: 0
From: IL
What is the part number for the 19 tooth driven gear you got from GM?
My chart shows a #3987919 Natural(white/cream). TCI chart shows same and lists a size of 2.25"L, .305" shaft dia., and .800" gear diameter. This is the same as a TH350
Based on your 27" tire (255/60/15"), 3.55 gear, 7 tooth drive (stock M21), your math is correct using a 19 tooth driven gear.
Later.
My chart shows a #3987919 Natural(white/cream). TCI chart shows same and lists a size of 2.25"L, .305" shaft dia., and .800" gear diameter. This is the same as a TH350
Based on your 27" tire (255/60/15"), 3.55 gear, 7 tooth drive (stock M21), your math is correct using a 19 tooth driven gear.
Later.
Thread Starter
Drifting
Joined: Sep 2004
Posts: 1,532
Likes: 4
From: Fairview Heights Illinois, near Saint Louis MO, STL C3 Shark
thanks guys, tomorrow Im working from home so I can stop by the speedo shop around my lunch break. I'll let everyone know what the outcome is.
.800 diameter? That is in between the 19tooth 3/4" (.75) andthe 22 tooth 7/8" (.875). Granted I don't have teh best of calipers to measure with
stay tuned..
.800 diameter? That is in between the 19tooth 3/4" (.75) andthe 22 tooth 7/8" (.875). Granted I don't have teh best of calipers to measure with
stay tuned..
Racer
Joined: Aug 2005
Posts: 435
Likes: 2
From: Boise Idaho
http://www.700r4.com/speedoCalc/speedocalc.shtml
This is a great tool for determining the Drive Gear and Driven Gear.
This is a great tool for determining the Drive Gear and Driven Gear.
Drifting
Joined: May 2004
Posts: 1,442
Likes: 2
From: Flanders NJ
Originally Posted by ruby76
My 4 speed 76 vette has a green 22 tooth driven gear. The speedometer is off, and I've done calculations and have found that I need a 19 tooth to even things out. I ordered a 19 tooth natural color driven gear from Chevrolet that came in today.
The green (original) driven gear is approx 1 7/8" diameter across the gear teeth. THe new one, is only 1 3/4" in diameter. I installed it not noticing and went for a test drive, got acouple of hundred feet from the house and noticed the speedo wasn't working at all.
Took it out, put it back in, figured I just didn't seat it right. No difference. PUlled it out, inpsected it, no stripped teeth, compared to the green on and noted the measurement difference above.
So, my problem - none of the online sites I find say that there even was a green, 22 tooth, 1 7/8" driven gear. Only a grey one.
In any event, where can I get the right 1 7/8" 19 tooth driven gear???
The green (original) driven gear is approx 1 7/8" diameter across the gear teeth. THe new one, is only 1 3/4" in diameter. I installed it not noticing and went for a test drive, got acouple of hundred feet from the house and noticed the speedo wasn't working at all.
Took it out, put it back in, figured I just didn't seat it right. No difference. PUlled it out, inpsected it, no stripped teeth, compared to the green on and noted the measurement difference above.
So, my problem - none of the online sites I find say that there even was a green, 22 tooth, 1 7/8" driven gear. Only a grey one.
In any event, where can I get the right 1 7/8" 19 tooth driven gear???
Thread Starter
Drifting
Joined: Sep 2004
Posts: 1,532
Likes: 4
From: Fairview Heights Illinois, near Saint Louis MO, STL C3 Shark
The "carrier" on my 4speed is not adjustable, but I see your point about loosing teeth and thus the diameter. I wonder if there is a different carrier on the 4speed for different driven gears/diameter.
I really hope I don't need to change drive gears - I'll let you guys know in a couple.
off to the speedo shop.....
I really hope I don't need to change drive gears - I'll let you guys know in a couple.
off to the speedo shop.....