Speedometer Accuracy Issues...Driven Gears
Thread Starter
Racer
Joined: Apr 2013
Posts: 302
Likes: 16
Speedometer Accuracy Issues...Driven Gears
1969 Corvette Muncie 4Spd
I have a 3.08 differential, 27" tires, and using a gray 22 tooth driven gear. My speedo is reading 40 mph but actual is 47mph.
When Speedo reads 60 mph the actual is 71 mph verified with gps.
I purchased a driven gear set ... 17 tooth to 22 tooth, I tried using the brown 18 tooth driven gear but made the speedometer read slower... 25 mph when actual was 50 mph, so 2 x the error. So I went back to the 22 tooth driven gear.
The kit didn't have 23 tooth gear... what can I do to get the speedo accurate?
I have a 3.08 differential, 27" tires, and using a gray 22 tooth driven gear. My speedo is reading 40 mph but actual is 47mph.
When Speedo reads 60 mph the actual is 71 mph verified with gps.
I purchased a driven gear set ... 17 tooth to 22 tooth, I tried using the brown 18 tooth driven gear but made the speedometer read slower... 25 mph when actual was 50 mph, so 2 x the error. So I went back to the 22 tooth driven gear.
The kit didn't have 23 tooth gear... what can I do to get the speedo accurate?
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,809
From: Cool Northern Michigan
You can never get a speedometer to read 100% accurate. So, you should always have it error on the high-side. It keeps you from getting (more) tickets.
Any time there is a drastic change in rear axle ratios, the "Drive" gear must be changed also. For example, going from a 4:56 rear to a 2:78.
Your speedo is off quite a bit, but I think its correctable by studying ratios at Speedometer World web site.
I could be wrong, but I think you need less teeth on the driven gear to spin faster.
Any time there is a drastic change in rear axle ratios, the "Drive" gear must be changed also. For example, going from a 4:56 rear to a 2:78.
Your speedo is off quite a bit, but I think its correctable by studying ratios at Speedometer World web site.
I could be wrong, but I think you need less teeth on the driven gear to spin faster.
Last edited by HeadsU.P.; Jun 14, 2020 at 04:28 PM.
Thread Starter
Racer
Joined: Apr 2013
Posts: 302
Likes: 16
You can never get a speedometer to read 100% accurate. So, you should always have it error on the high-side. It keeps you from getting (more) tickets.
Any time there is a drastic change in rear axle ratios, the "Drive" gear must be changed also. For example, going from a 4:56 rear to a 2:78.
Your speedo is off quite a bit, but I think its correctable by studying ratios at Speedometer World web site.
I could be wrong, but I think you need less teeth on the driven gear to spin faster.
Any time there is a drastic change in rear axle ratios, the "Drive" gear must be changed also. For example, going from a 4:56 rear to a 2:78.
Your speedo is off quite a bit, but I think its correctable by studying ratios at Speedometer World web site.
I could be wrong, but I think you need less teeth on the driven gear to spin faster.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,809
From: Cool Northern Michigan
Try TCI Automotive (speedometer world). They have calculators, gear color codes, tooth count, etc.
Hope that a correction can be done easily at the trans driven gear. If you have to swap out the Drive gear, not a fun job. Involves driveshaft / tailshaft housing removal.
Need small hands to unclip old gear, slide off of tailshaft, etc, etc, etc.
Hope that a correction can be done easily at the trans driven gear. If you have to swap out the Drive gear, not a fun job. Involves driveshaft / tailshaft housing removal.
Need small hands to unclip old gear, slide off of tailshaft, etc, etc, etc.
Race Director
Joined: May 2006
Posts: 18,598
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Exactly what size try do you have, 225/70x15 etc.?
Have you actually counted the number of driveshaft revolutions per 1 tire revolution to confirm the 3.08?
The 22 tooth gear is used for 3.55 and 3.73 depending on tire size.
Have you actually counted the number of driveshaft revolutions per 1 tire revolution to confirm the 3.08?
The 22 tooth gear is used for 3.55 and 3.73 depending on tire size.
Last edited by MelWff; Jun 14, 2020 at 06:25 PM.
Is my vette stock?? HAHA
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From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
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Here’s a thread with some links. Thegearbox.org has gears with more teeth but it could be a problem with the drive gear. Some tranny have plastic gears and they can slip....just a thought.
https://www.corvetteforum.com/forums...post1601465889
when you scroll down there’s a calculator for figuring what should work
https://www.corvetteforum.com/forums...post1601465889
when you scroll down there’s a calculator for figuring what should work
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Team Owner
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From: Crossville TN
You need fewer teeth on the driven speedo gear to make the speedometer read 'faster'. For the amount of error you have, I would go with 3 fewer teeth than what you have now.
These gears come in "sets". For each 'drive' gear, there are several 'driven' gears that will work with it; and each drive gear MUST have driven gears within a specific range of teeth. You can't go any more or less than what the drive gear will allow. I'm pretty sure that you can get a 19 tooth driven gear for your 'set', but you need to research that to be sure.
These gears come in "sets". For each 'drive' gear, there are several 'driven' gears that will work with it; and each drive gear MUST have driven gears within a specific range of teeth. You can't go any more or less than what the drive gear will allow. I'm pretty sure that you can get a 19 tooth driven gear for your 'set', but you need to research that to be sure.
Le Mans Master
Joined: Oct 2010
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From: melbourne florida
2023 C3 of the Year Finalist - Modified
California performance transmissions has a speedometer corrector you take a reading at an indicated 55 miles an hour on your speedometer and read what the GPS on your phone says and they will make you a box that will correct the speed
Le Mans Master
Joined: Oct 2010
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From: melbourne florida
2023 C3 of the Year Finalist - Modified
the other option is jacking the car up getting under their taking the gear out counting the teeth putting it back in putting the car back down ordering the gear putting it in after jacking the car up again this way all you do is Jack the car up once put the bucks in put the car down done you pay for convenience
Thread Starter
Racer
Joined: Apr 2013
Posts: 302
Likes: 16
I have a 3.08 gear. I recently went from a 3.90 to a 3.08 differential.
There was a gray 22 tooth driven gear in the tail shaft.
actual mph was higher than the speedo . See 1st post..
I purchased a driven gear set and tried a Brown 18 tooth gear. At 50mph actual the speedo then reads 25mph! so the speedo read slower with the lower tooth gear.
Last edited by Z06CE; Jun 15, 2020 at 09:07 PM.
Race Director
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There is no 3.90 listed for a 69 4 sp.
For the 3.08 the only driven gear listed is the 18 tooth brown gear.
Can you see the drive gear, is it damaged or slipping?
For the 3.08 the only driven gear listed is the 18 tooth brown gear.
Can you see the drive gear, is it damaged or slipping?
Last edited by MelWff; Jun 15, 2020 at 09:08 PM.
Thread Starter
Racer
Joined: Apr 2013
Posts: 302
Likes: 16
Someone other than me installed the 390 with the 22 tooth gear. I didnt like the 390 when I purchased the car. Thats why I put the 308 in the car. I think the speedo is out of calibration.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
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From: Cool Northern Michigan
Just for clarification, the "Drive Gear" is mounted on the tailshaft. It drives the Driven Gear which has easy access at the end of the cable.
The Drive Gear is more difficult to swap out.
I believe there are only a few Drive gears to cover a wide array (dozens) of Driven Gears choices..
You really need to know the tooth count on the Drive Gear so you know what you are dealing with. It is possible to count the teeth of the Drive G. with a flashlight in the hole.
Tough, but doable. You can also rotate the driveshaft if the tires are off the ground and count that gears teeth.
The Drive Gear is more difficult to swap out.
I believe there are only a few Drive gears to cover a wide array (dozens) of Driven Gears choices..
You really need to know the tooth count on the Drive Gear so you know what you are dealing with. It is possible to count the teeth of the Drive G. with a flashlight in the hole.
Tough, but doable. You can also rotate the driveshaft if the tires are off the ground and count that gears teeth.
Last edited by HeadsU.P.; Jun 16, 2020 at 10:00 AM.
Thread Starter
Racer
Joined: Apr 2013
Posts: 302
Likes: 16
Just for clarification, the "Drive Gear" is mounted on the tailshaft. It drives the Driven Gear which has easy access at the end of the cable.
The Drive Gear is more difficult to swap out.
I believe there are only a few Drive gears to cover a wide array (dozens) of Driven Gears choices..
The Drive Gear is more difficult to swap out.
I believe there are only a few Drive gears to cover a wide array (dozens) of Driven Gears choices..
Thread Starter
Racer
Joined: Apr 2013
Posts: 302
Likes: 16
I haven't been able to see the drive gear. I have to jack the car up and try to use a mirror to see the drive gear.