79 speedo gear question
Thread Starter
Racer
Joined: Oct 2006
Posts: 412
Likes: 1
From: St. John's Newfoundland
79 speedo gear question
Can anyone tell me the revolutions per mile our speedo cable turns?
I have a 2004R transmission with 255/60/15 tires with a gear ratio 3.55:1.
I installed a 10T (green) drive gear and a 27 T (black) driven gear last year when I did the 2004R swap.
The problem I have is that when my GPS indicates 100 KM/HR, my speedo is reading 120 KM/HR. Assuming that our speedometer cable rotate 1000 revolutions per mile this should be correct, according to the Speedo Gear Calculator from TCI. I have to go into the transmission to install the lock-up wiring harness so I was hoping to solve both problems the same time. Any suggestions would be appreciated.
I have a 2004R transmission with 255/60/15 tires with a gear ratio 3.55:1.
I installed a 10T (green) drive gear and a 27 T (black) driven gear last year when I did the 2004R swap.
The problem I have is that when my GPS indicates 100 KM/HR, my speedo is reading 120 KM/HR. Assuming that our speedometer cable rotate 1000 revolutions per mile this should be correct, according to the Speedo Gear Calculator from TCI. I have to go into the transmission to install the lock-up wiring harness so I was hoping to solve both problems the same time. Any suggestions would be appreciated.
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,851
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
Can anyone tell me the revolutions per mile our speedo cable turns?
I have a 2004R transmission with 255/60/15 tires with a gear ratio 3.55:1.
I installed a 10T (green) drive gear and a 27 T (black) driven gear last year when I did the 2004R swap.
The problem I have is that when my GPS indicates 100 KM/HR, my speedo is reading 120 KM/HR. Assuming that our speedometer cable rotate 1000 revolutions per mile this should be correct, according to the Speedo Gear Calculator from TCI. I have to go into the transmission to install the lock-up wiring harness so I was hoping to solve both problems the same time. Any suggestions would be appreciated.
I have a 2004R transmission with 255/60/15 tires with a gear ratio 3.55:1.
I installed a 10T (green) drive gear and a 27 T (black) driven gear last year when I did the 2004R swap.
The problem I have is that when my GPS indicates 100 KM/HR, my speedo is reading 120 KM/HR. Assuming that our speedometer cable rotate 1000 revolutions per mile this should be correct, according to the Speedo Gear Calculator from TCI. I have to go into the transmission to install the lock-up wiring harness so I was hoping to solve both problems the same time. Any suggestions would be appreciated.
Thread Starter
Racer
Joined: Oct 2006
Posts: 412
Likes: 1
From: St. John's Newfoundland
I'll assume if 30mph is 500 rpm, 60 mph should be 1000 rpm, I'll use that in the calculator.
Last edited by cboyd; Jan 13, 2015 at 06:16 PM.
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,851
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
Yes.. this comes to you directly from my old Borg speedo/tach calibration machine.. lol.. it is a linear scale.
The machine has three scales. 1) MPH, 2) Rev @ .50 3) rev @ 1-1. 2 and 3 are used when calibrating the tachometers, one is for distributor drive and the other for generator drive. Its a very cool and neat old machine from the 1960's.
Also keep in mind that 1 tooth per 5 mph is a close.. (not exact) standard for going up or down to dial in the speedo.
Willcox
The machine has three scales. 1) MPH, 2) Rev @ .50 3) rev @ 1-1. 2 and 3 are used when calibrating the tachometers, one is for distributor drive and the other for generator drive. Its a very cool and neat old machine from the 1960's.
Also keep in mind that 1 tooth per 5 mph is a close.. (not exact) standard for going up or down to dial in the speedo.
Willcox
Last edited by Willcox Corvette; Jan 13, 2015 at 09:06 PM.
Thread Starter
Racer
Joined: Oct 2006
Posts: 412
Likes: 1
From: St. John's Newfoundland
Yes.. this comes to you directly from my old Borg speedo/tach calibration machine.. lol.. it is a linear scale.
The machine has three scales. 1) MPH, 2) Rev @ .50 3) rev @ 1-1. 2 and 3 are used when calibrating the tachometers, one is for distributor drive and the other for generator drive. Its a very cool and neat old machine from the 1960's.
Also keep in mind that 1 tooth per 5 mph is a close.. (not exact) standard for going up or down to dial in the speedo.
Willcox
The machine has three scales. 1) MPH, 2) Rev @ .50 3) rev @ 1-1. 2 and 3 are used when calibrating the tachometers, one is for distributor drive and the other for generator drive. Its a very cool and neat old machine from the 1960's.
Also keep in mind that 1 tooth per 5 mph is a close.. (not exact) standard for going up or down to dial in the speedo.
Willcox
Melting Slicks
Joined: Oct 2006
Posts: 2,417
Likes: 336
From: Texas
try this calculator, i have the same speedometer gears as you. close as i could get. couldnt find a 29 tooth one.
http://www.montess.org/speedo_gears.html
http://www.montess.org/speedo_gears.html
Thread Starter
Racer
Joined: Oct 2006
Posts: 412
Likes: 1
From: St. John's Newfoundland
try this calculator, i have the same speedometer gears as you. close as i could get. couldnt find a 29 tooth one.
http://www.montess.org/speedo_gears.html
http://www.montess.org/speedo_gears.html
Melting Slicks
Joined: Oct 2006
Posts: 2,417
Likes: 336
From: Texas
calculated out to .377, i have the green drive gear also, all i found at the time was a driven29. so mine is off some too. a 27 would probably closer to correct ratio. my speedo is off about 4 mph.
http://www.ebay.com/itm/27-tooth-Black-2004R-GM-Speedometer-gear-/151164777716?pt=Motors_Car_Truck_Parts_Accessories&hash=item23321f74f4&vxp=mtr
http://www.ebay.com/itm/27-tooth-Black-2004R-GM-Speedometer-gear-/151164777716?pt=Motors_Car_Truck_Parts_Accessories&hash=item23321f74f4&vxp=mtr
Last edited by speedreed8; Jan 14, 2015 at 10:44 AM.
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Thread Starter
Racer
Joined: Oct 2006
Posts: 412
Likes: 1
From: St. John's Newfoundland
calculated out to .377, i have the green drive gear also, all i found at the time was a driven29. so mine is off some too. a 27 would probably closer to correct ratio. my speedo is off about 4 mph.
27 Tooth Black 2004R GM Speedometer Gear | eBay
27 Tooth Black 2004R GM Speedometer Gear | eBay
Thanks
Clyde
Thread Starter
Racer
Joined: Oct 2006
Posts: 412
Likes: 1
From: St. John's Newfoundland
Clyde
Thread Starter
Racer
Joined: Oct 2006
Posts: 412
Likes: 1
From: St. John's Newfoundland
Clyde
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,851
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
The 1 tooth per 5 is a long time standard that has been used for a very long time..
But.. this is based on having the correct tranny in the car and making a rear swap, not a tranny swap. Will this hold true to your 200R4 swap... I'll be honest, I have not a clue.
What you have to keep in check... Drive gears will only mesh up to a fixed number of driven gears. So exceeding the limits of the drive gear match will result in a failure of both gears in most cases.
What about one of the ratio adapters.. This might be a solution to your issue too..
Good Luck... I hope you figure it out.
Willcox
But.. this is based on having the correct tranny in the car and making a rear swap, not a tranny swap. Will this hold true to your 200R4 swap... I'll be honest, I have not a clue.
What you have to keep in check... Drive gears will only mesh up to a fixed number of driven gears. So exceeding the limits of the drive gear match will result in a failure of both gears in most cases.
What about one of the ratio adapters.. This might be a solution to your issue too..
Good Luck... I hope you figure it out.
Willcox
Last edited by Willcox Corvette; Jan 14, 2015 at 10:10 PM.
Safety Car
Joined: Apr 2006
Posts: 4,057
Likes: 421
From: BC
Yes.. this comes to you directly from my old Borg speedo/tach calibration machine.. lol.. it is a linear scale.
The machine has three scales. 1) MPH, 2) Rev @ .50 3) rev @ 1-1. 2 and 3 are used when calibrating the tachometers, one is for distributor drive and the other for generator drive. Its a very cool and neat old machine from the 1960's.
Also keep in mind that 1 tooth per 5 mph is a close.. (not exact) standard for going up or down to dial in the speedo.
Willcox
The machine has three scales. 1) MPH, 2) Rev @ .50 3) rev @ 1-1. 2 and 3 are used when calibrating the tachometers, one is for distributor drive and the other for generator drive. Its a very cool and neat old machine from the 1960's.
Also keep in mind that 1 tooth per 5 mph is a close.. (not exact) standard for going up or down to dial in the speedo.
Willcox
Thanks.
Thread Starter
Racer
Joined: Oct 2006
Posts: 412
Likes: 1
From: St. John's Newfoundland
The 1 tooth per 5 is a long time standard that has been used for a very long time..
But.. this is based on having the correct tranny in the car and making a rear swap, not a tranny swap. Will this hold true to your 200R4 swap... I'll be honest, I have not a clue.
What you have to keep in check... Drive gears will only mesh up to a fixed number of driven gears. So exceeding the limits of the drive gear match will result in a failure of both gears in most cases.
What about one of the ratio adapters.. This might be a solution to your issue too..
Good Luck... I hope you figure it out.
Willcox
But.. this is based on having the correct tranny in the car and making a rear swap, not a tranny swap. Will this hold true to your 200R4 swap... I'll be honest, I have not a clue.
What you have to keep in check... Drive gears will only mesh up to a fixed number of driven gears. So exceeding the limits of the drive gear match will result in a failure of both gears in most cases.
What about one of the ratio adapters.. This might be a solution to your issue too..
Good Luck... I hope you figure it out.
Willcox
I am kinda waiting for someone to chime in that has had this issue and how they solved it.
I drive and driven gears are correct, I have to wonder if it is in the speedo itself. Won't be able to deal with that until spring. Thanks again.
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,851
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
I have been trying to find on line which combinations will work together, but from what I can see, it just list driven gears, and drive gears.
I am kinda waiting for someone to chime in that has had this issue and how they solved it.
I drive and driven gears are correct, I have to wonder if it is in the speedo itself. Won't be able to deal with that until spring. Thanks again.
I am kinda waiting for someone to chime in that has had this issue and how they solved it.
I drive and driven gears are correct, I have to wonder if it is in the speedo itself. Won't be able to deal with that until spring. Thanks again.
If so, you could possibly make an adjustment in the speedometer. So knowing the RPM vs. speed would be handy for that. The speed cup inside the speedo has a spring on it that is adjustable. Pushing the adjustment lever down increases the spring drag (slows down) and raising it decreases it.
This is how we calibrate the speedometers... So if you know how far you are off at say 60 (1000 rpms) then you'd need to find something to drive the speedo 1000 rpms while you adjust the spring...
Food for thought.
Willcox
Melting Slicks
Joined: Oct 2006
Posts: 2,417
Likes: 336
From: Texas
speedometer gear calculator. calculates that i should run a 10/26, but i have a 10/29. oh well.....so close.
http://tri-5.chevyrides.com/speedo.php
http://tri-5.chevyrides.com/speedo.php
Thread Starter
Racer
Joined: Oct 2006
Posts: 412
Likes: 1
From: St. John's Newfoundland
Is the difference in the speed linear over the scale?
If so, you could possibly make an adjustment in the speedometer. So knowing the RPM vs. speed would be handy for that. The speed cup inside the speedo has a spring on it that is adjustable. Pushing the adjustment lever down increases the spring drag (slows down) and raising it decreases it.
This is how we calibrate the speedometers... So if you know how far you are off at say 60 (1000 rpms) then you'd need to find something to drive the speedo 1000 rpms while you adjust the spring...
Food for thought.
Willcox
If so, you could possibly make an adjustment in the speedometer. So knowing the RPM vs. speed would be handy for that. The speed cup inside the speedo has a spring on it that is adjustable. Pushing the adjustment lever down increases the spring drag (slows down) and raising it decreases it.
This is how we calibrate the speedometers... So if you know how far you are off at say 60 (1000 rpms) then you'd need to find something to drive the speedo 1000 rpms while you adjust the spring...
Food for thought.
Willcox
Thread Starter
Racer
Joined: Oct 2006
Posts: 412
Likes: 1
From: St. John's Newfoundland
speedometer gear calculator. calculates that i should run a 10/26, but i have a 10/29. oh well.....so close.
http://tri-5.chevyrides.com/speedo.php
http://tri-5.chevyrides.com/speedo.php