79 Speedo reads too slow.
Thread Starter
Racer
Joined: Aug 2009
Posts: 363
Likes: 72
From: Somers New York
79 Speedo reads too slow.
I have a 1979 L-82 4 speed that originally came with a 3.70 rear gear ratio. At some point in the past 32 years, a 3.08 diff was installed. According to my GPS, I was traveling approximately 12 MPH faster than the speedo indicated. Example: 30MPH indicated would be 42MPH as per my GPS. The driven gear in the trans had 20 teeth. An online speedometer calculator recommended an 18 tooth gear. I just installed that this weekend and while it helped some, the speedo still runs about 8 MPH slower. I believe 18 tooth is the smallest gear available. Do I have any other options, besides looking like a tourist with a GPS stuck to my windshield?
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,853
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
I have a 1979 L-82 4 speed that originally came with a 3.70 rear gear ratio. At some point in the past 32 years, a 3.08 diff was installed. According to my GPS, I was traveling approximately 12 MPH faster than the speedo indicated. Example: 30MPH indicated would be 42MPH as per my GPS. The driven gear in the trans had 20 teeth. An online speedometer calculator recommended an 18 tooth gear. I just installed that this weekend and while it helped some, the speedo still runs about 8 MPH slower. I believe 18 tooth is the smallest gear available. Do I have any other options, besides looking like a tourist with a GPS stuck to my windshield?
About all you can do is change the driven gear and that is a real PITA.
Once you reach the limit then you're only option is change the driven gear or find a ratio coupler (adapter). You might want to look online and see if you can find one. I suppose I could tweak the calibration on the speedo too.
Willcox
Instructor
Joined: Sep 2007
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From: Colorado Springs Colorado
Burning Brakes
Joined: Jun 2011
Posts: 817
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From: lewisburg wv
Former Vendor
Joined: Aug 2006
Posts: 76,656
Likes: 1,853
From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
This vendor sells a 17 tooth driven gear.
http://www.thegearbox.org/home.html
http://www.thegearbox.org/home.html
The 3:70 used a 7 tooth drive gear with a 20 tooth blue driven gear.
The 3:08 used a 8 tooth drive gear with a 19 tooth natural driven gear.
You'll find the GM factory chart at this link:
http://willcoxcorvette.com/repairand...lp.php?hID=259
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
To get it working well, you will have to change out the speedo "drive" gear in your transmission, as well as the driven gear. You can go to the TCI website 'speedo gear calculator' and determine what driven gear you need...along with the 8 tooth drive gear...to get an accurate speedo reading. To change the drive gear, you have to pull the rear housing off the back of the transmission, depress the speedo gear retaining clip, and slide the 'old' gear off...then slide the new gear on over the clip and release it. You should have a new tailshaft gasket available, in case the old one tears or is not serviceable when you disassemble it. I don't have a manual trans, so I can't tell you what problems you will have removing [or disconnecting] the driveshaft, but the yoke and tailshaft housing removal shouldn't be a problem. Since you do not have a VSS type electronic speedometer, you can't use a "ratio adapter" to fix your problem; so the internal gear change is the only way [I know of] to get the job done right.
{Bubba would use some sticky notes to create a new MPH scale for his speedometer, but I don't think that will do...
}
{Bubba would use some sticky notes to create a new MPH scale for his speedometer, but I don't think that will do...
}
Instructor
Joined: Sep 2007
Posts: 198
Likes: 0
From: Colorado Springs Colorado
7T1vette,
If you open this link:
http://www.redrider.us/Speedometer_G...atio%20adapter
You'll see a link for 'Ratio Adapters' (second link in blue), As Wilcox pointed out, you can use this cable adapter to arrive at almost any adjustment needed to get your speedo reading correctly. They even have custom gearing to get it spot on. Cost less than $75.00. Now you know of two way's to get the job done right.
http://www.redrider.us/Speedometer_G...atio%20adapter
You'll see a link for 'Ratio Adapters' (second link in blue), As Wilcox pointed out, you can use this cable adapter to arrive at almost any adjustment needed to get your speedo reading correctly. They even have custom gearing to get it spot on. Cost less than $75.00. Now you know of two way's to get the job done right.
Last edited by tonyv123; Jul 11, 2011 at 01:12 PM.
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Since I really don't want to split the tranny case, the adapter route looks like the way I'll go.
Nice to know...
