Ken79

I have an all original, 79 L82 4-speed, 3.36 gears, that I ordered/purchased new. The speedo has read too low since day 1 - it reads about 70 when running at 75 actual mph. I pulled out the original speedo gear and found it to be the 20 tooth blue one. Could not find one at Carlisle, as the vendors do not bring these simple things there. I'm getting ready to order one from one of the usual parts catalogs. After looking at many catalogs, most do not tell you how many mph each tooth change represents. But, in 2 vendor catalogs that do tell you the mph impact, they conflict. One catalog says a 1 tooth change = 1.5 mph change. Another catalog says a 1 tooth change = 5 mph change. I know I need to go from 20 teeth to fewer teeth. Both catalogs cannot be correct. There is a 19 tooth natural color, and a 18 tooth brown color, available. Problem is, which one to choose?

Can someone advise with certainty on how many mph 1 gear tooth tooth represents?

Thanks, Ken

Edit - I found this thread on the same problem in another 79 -
http://forums.corvetteforum.com/c3-t...-too-slow.html

The Speedo Gear chart from Willcox tells me I should have a code B (green) drive gear, and tells me I should have a code N (brown, 18 teeth) driven gear. (From the factory, mine came with the blue, 20 teeth gear).

Looks like I need to go with a brown, 18 tooth driven gear to resolve my problem. Can I get confirmation that I'm headed in the right direction?

pawpaws toy

If the drive gear is the green 7 tooth then the correct driven gear is the brown 18t The blue gear 20t is for the 3.08 rear.

0Willcox Corvette

I just looked at the chart and that should do you. How far is the speedo off? I'm not sure where the info came from.. but 1 tooth is about 5 mph.

Ken79

I did not notice the color of the drive gear or the number of teeth when I pulled the speedo cable out of the tranny to determine the color of the driven gear, which is blue 20T.

In the Willcox chart from the above noted other thread, there is not a 3.08 rear listed for 79.

Ken79

When the speedo reads 70, I'm actually going 75-76. If each tooth represents a 5 mph impact, then I should go to the 19T Natural colored gear. This would not be in agreement with the Willcox speedo gear usage chart, which advises to use 18T Brown gear.

PS - I'm certain I have a 3.36 rear, and the tires are the original sized 225/70-15. The 3.36 rear came in those L82 cars that were ordered with the RPO G95 Hi-Way axle, which I did. I have also confirmed the 3.36 by the driveshaft/wheel rotation approach.

Vette5.5

Probably best keeping things in simple percetages. In your case, about 8% too low of reading. Think I'd start by a cable driven gear with 8% less teeth. Things can get complicated if going to far from what you started with though. First would be a different cable driven gear mount, and maybe pulling the tranny tail shaft housing to change the drive gear. You'll know if you have the wrong driven gear for the housing, as either wont fit, or will slop around. At 8%, you should be fine, but could already gear sized at the end of the mount range. Really no big deal, as the mountings pop right in.

BKbroiler

You can't equate number of teeth to a number of miles per hour. If a one tooth count difference changed the speedo reading by 6 miles per hour at 60 mph, it would only change the reading by 3 mph at 30. The same percentage change (10%) each time.

Ken79

Yes, I plan to keep the resolution simple. After all, I've lived with this speedo error for 32 years and 22K miles !!!

Bottom line here - I'm only going to install a new driven gear for this "problem". Therefore, the only question is - should it be 18T Brown, or 19T Natural gear?

0Willcox Corvette

Ken, I would go with 1 tooth for now and see what the results are. The speedometer itself has a margin for error and for 1978-1982 I'd have to look it up tomorrow to be sure what it is.. I think it is 1-3 mph. The only book I have here at home is from 1968

'75

While the experts are here, my friend has a 79 that originally had 3.55 gears and now has 4.11, which driven gear should be used? Or what drive gear did the car come with?

78 silver 78

I would say, it's not that simple a question. What is your drive gear? You can use the speedometer calibration program at http://www.redrider.us/speedometer gear.htm but you need to input some info.

78 silver 78

GM 4 speed w/ 225/70-15 , 3.36 rear, would have had 8 tooth drive / 20 tooth driven.

78 silver 78

what size tires , auto or four speed ?

7T1vette

18 tooth [brown] gear will get the job done.

Ken79

Tried to follow that link, but it is broken.

pawpaws toy

The calculator I checked stated the drive gear would also need to be changed with 4.11 gears

'75

TH350, 255/60/15 tires.
Thanks

78 silver 78

Stock 3.55 would have had 8 orange / 22 gray . You would need 7 drive / 22 driven for 4.11

'75

Thanks

78 silver 78

Do you have cruise control ? If so, could something be wrong with the transducer ?