speedo gear
Thread Starter
Burning Brakes
Joined: Feb 2014
Posts: 1,045
Likes: 226
From: Cape Neddick Maine
2020 C3 of the Year Finalist - Modified
speedo gear
I have a 76 Vette that has a turbo 350 and 3.73 gears. does anyone know what gear I need for the transmission to make the speedo read correctly? I am not sure what the original rear set was or remember at this time what the tach was reading going down the road or how far off the speedo was.
If memory serves me right I am thinking it may have been off like 10 mph at 60,so it was reading around 70.
thanks for the support.
John
If memory serves me right I am thinking it may have been off like 10 mph at 60,so it was reading around 70.
thanks for the support.
John
2026 Loser of the Year
Joined: Sep 2013
Posts: 36,559
Likes: 7,006
From: New Or-leens Loo-z-anna
Willcox has a chart showing which color speedometer gear should go in the transmission. You match your differential gear ratio to your type of transmission to determine which plastic gear you need. If you have tires which are a smaller diameter than the recommended tires, you may need to buy the next-lower-tooth gear
Thread Starter
Burning Brakes
Joined: Feb 2014
Posts: 1,045
Likes: 226
From: Cape Neddick Maine
2020 C3 of the Year Finalist - Modified
awesome info thank you.
I spent the day with my Mom and Dad 72 and 75 trying to get ready for another winter storm. My folks are afraid of losing power and not having heat. We got an old wood stove that was converted to kerosene running just in case. It was my great-grandmothers stove LOL. It is cast iron and believe it was a Maggie
I spent the day with my Mom and Dad 72 and 75 trying to get ready for another winter storm. My folks are afraid of losing power and not having heat. We got an old wood stove that was converted to kerosene running just in case. It was my great-grandmothers stove LOL. It is cast iron and believe it was a Maggie
2026 Loser of the Year
Joined: Sep 2013
Posts: 36,559
Likes: 7,006
From: New Or-leens Loo-z-anna
That was a very good thing to do....nothing like having dependable spare heat in your part of the country. And your parents will sleep better at night knowing they're taken care of in those storms.
The speedometer gear chart shows I need to go two gear-sizes higher than what the previous owner installed, since my speedometer is 20mph too high at 60 actual/gps mph.
I read in another thread that 60 mph showing on the speedometer/70 mph showing on the gps means you need a gear with less teeth. If the speedometer showed 70 mph/gps showed 60 mph, you would need a gear with more teeth (I think I said that correctly)
The speedometer gear chart shows I need to go two gear-sizes higher than what the previous owner installed, since my speedometer is 20mph too high at 60 actual/gps mph.
I read in another thread that 60 mph showing on the speedometer/70 mph showing on the gps means you need a gear with less teeth. If the speedometer showed 70 mph/gps showed 60 mph, you would need a gear with more teeth (I think I said that correctly)
How far are you off? A general rule of thumb is 1 tooth per 5 mph. The only issue is if you run past the mesh range of the drive gear inside the transmission. Someone posted a calculator page here that was for all cars a while back, maybe someone will see this and post it again. Willcox 
Last edited by doorgunner; Feb 15, 2015 at 05:20 PM.
Le Mans Master
Joined: May 2002
Posts: 5,084
Likes: 786
From: Lebanon Township New Jersey
If the speedo is reading 60 when the car is actually doing 70, you do need a driven speedo gear with fewer teeth. In this case 60/70 = .86 So you need to count the teeth on the gear now and multiply by .86 to get the new number required. For example 22 teeth X .86 = 18.92, or say 19.
Drifting
Joined: Apr 2008
Posts: 1,305
Likes: 2
From: Lewisburg WV
thegazman
The rule of thumb for speedometer driven gears is one tooth equals five miles per hour. Keep in mind the drive gear has a mesh range. In other words your drive gear will only mesh with a given number of teeth on the driven gear before it has to be changed.
Le Mans Master
Joined: May 2002
Posts: 5,084
Likes: 786
From: Lebanon Township New Jersey
One tooth equals 5 mph, at what actual speed? If your speedo is off 5 mph at an actual speed of 20 mph the change in tooth count needed is different than if the speed is off 5 mph at 60. Its all trial and error unless yoe do it the way I described.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
10 Most Common Corvette Problems of the Last 20 Years!
Joe Kucinski
Thread Starter
Burning Brakes
Joined: Feb 2014
Posts: 1,045
Likes: 226
From: Cape Neddick Maine
2020 C3 of the Year Finalist - Modified
Thanks for all the information! We had another snow attack yesterday dropping about 21.0 inches along the seacoast. Hopefully the snow will melt sometime this year so I can start working on the car again.
I have a lot of work to do this year, rebuilding the front suspension, rear trailing arms/and bearings. I have had a pesky rear main leaking so the engine is coming out to be freshened up and totally rebuilt so I will take care of the speedo at this time. Damn it sounds like this is a real s-it box needing a lot LOL
I have a lot of work to do this year, rebuilding the front suspension, rear trailing arms/and bearings. I have had a pesky rear main leaking so the engine is coming out to be freshened up and totally rebuilt so I will take care of the speedo at this time. Damn it sounds like this is a real s-it box needing a lot LOL
Drifting
Joined: Apr 2008
Posts: 1,305
Likes: 2
From: Lewisburg WV
thegazman
I don't guess you know what "rule of thumbs"means. You mean to tell me you use a driven gear around town then change it if you are going to drive on the interstate. I got that rule of thumb from Willcox Corvette. Plus, the speedometer is not direct drive and does some adjusting for different speeds by design.
Le Mans Master
Joined: May 2002
Posts: 5,084
Likes: 786
From: Lebanon Township New Jersey
I don't guess you know what "rule of thumbs"means. You mean to tell me you use a driven gear around town then change it if you are going to drive on the interstate. I got that rule of thumb from Willcox Corvette. Plus, the speedometer is not direct drive and does some adjusting for different speeds by design.
Drifting
Joined: Apr 2008
Posts: 1,305
Likes: 2
From: Lewisburg WV
thegazman
BK... I concede you are a great mathematician and your mathematics is logical. However, a speedometer is similar to a centrifugal clutch in that the speed indicator dial varies depending upon the speed of the speedometer cable making it difficult calculate the required number driven gear teeth. I can only tell you what I know from experience. I had my differential rebuilt with a different gear ratio and I changed my driven gear several times to obtain a close to accurate speedometer reading at 60 MPH using a GPS. I guarantee you changing from a 22 to 19 tooth driven gear will change more than 10 MPH.
Maybe I do need a mathematics lesson as the suggest; however, one thing I am not lacking is common since.
Maybe I do need a mathematics lesson as the suggest; however, one thing I am not lacking is common since.
Le Mans Master
Joined: May 2002
Posts: 5,084
Likes: 786
From: Lebanon Township New Jersey
BK... I concede you are a great mathematician and your mathematics is logical. However, a speedometer is similar to a centrifugal clutch in that the speed indicator dial varies depending upon the speed of the speedometer cable making it difficult calculate the required number driven gear teeth. I can only tell you what I know from experience. I had my differential rebuilt with a different gear ratio and I changed my driven gear several times to obtain a close to accurate speedometer reading at 60 MPH using a GPS. I guarantee you changing from a 22 to 19 tooth driven gear will change more than 10 MPH.
Maybe I do need a mathematics lesson as the suggest; however, one thing I am not lacking is common since.
Maybe I do need a mathematics lesson as the suggest; however, one thing I am not lacking is common since.
22/19 = 1.16 so the cable will rotate 16% faster than it did with the 22 tooth gear. So when the speedo was reading 60 with a 22 tooth gear, it will now read about 70 (60 x 1.16 = 69.6) with the 19 tooth. If this doesn't make sense, we can agree to disagree.
Drifting
Joined: Apr 2008
Posts: 1,305
Likes: 2
From: Lewisburg WV
thegazman
gazman - I don't doubt your experience level and I understand the "rule of thumb" concept. I was trying to explain the method of determining what gear is needed, and only having to do it once. (BTW - I changed from 3.08 rear to 3.73 in my car and used my method, so it works.) I understand what you're saying about the speedo operation itself, but we are not changing anything in the speedo, just the speed of the cable in rpms. If you double the speed of the cable, the speedo will read twice as high. Regarding your example of 60 mph with a 22 tooth gear then changing to a 19 tooth gear:
22/19 = 1.16 so the cable will rotate 16% faster than it did with the 22 tooth gear. So when the speedo was reading 60 with a 22 tooth gear, it will now read about 70 (60 x 1.16 = 69.6) with the 19 tooth. If this doesn't make sense, we can agree to disagree.
22/19 = 1.16 so the cable will rotate 16% faster than it did with the 22 tooth gear. So when the speedo was reading 60 with a 22 tooth gear, it will now read about 70 (60 x 1.16 = 69.6) with the 19 tooth. If this doesn't make sense, we can agree to disagree.
Melting Slicks
Joined: Nov 2000
Posts: 2,507
Likes: 114
From: St. Louis Mo
I have a 76 Vette that has a turbo 350 and 3.73 gears. does anyone know what gear I need for the transmission to make the speedo read correctly? I am not sure what the original rear set was or remember at this time what the tach was reading going down the road or how far off the speedo was.
If memory serves me right I am thinking it may have been off like 10 mph at 60,so it was reading around 70.
thanks for the support.
John
If memory serves me right I am thinking it may have been off like 10 mph at 60,so it was reading around 70.
thanks for the support.
John
2026 Loser of the Year
Joined: Sep 2013
Posts: 36,559
Likes: 7,006
From: New Or-leens Loo-z-anna
Le Mans Master
Joined: May 2002
Posts: 5,084
Likes: 786
From: Lebanon Township New Jersey
BK..... Yes, that does make sense. I also wound up changing my drive gear due to wear. I laugh to myself because I went from a 3.73 to a 3.36 as I disliked the high RPM on the interstate and you changed to a 3.73. We could have traded differentials. HA! Thanks for being patient with me. I'm getting old and hardheaded.
BTW, I'm pretty old myself.