Speedo Gear Question

[Ken79]

I have an all original, 79 L82 4-speed, 3.36 gears, that I 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

 

[bill81vette]

Ken,check this out ...
hope it helps...
Bill...Speedometer Gear Calculator

 

[JohnZ]

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

Thanks, Ken

One tooth difference is normally a 5% change; 5% at 70 is 3.5 mph.

 

[Ken79]

Can't use the calculators effectively as I do not know the actual drive gear (7T or 8T) that was installed in the trans at the factory.

Ordered both 18T & 19T and will go with the one that produces the best results.

 

[Peer81]

I had the same problem. What I did is drive around with a navigation that also has a mph indication and check them as some speeds like in town, out town and on highway (20, 40, 60?) Then also write down the speedometer speeds. If you also know the drive and driven gear colors you have right now I can give you the exact color gear you need. As the speedo isn't acurate over the speed range you also need to choose if you want it more acurate on the low side or on the high side (what speed you drive the most for instance).

Greetings Peter

 

[GTR1999]

Hi Ken,
Not sure about the 79's but I know that the 75's we had/have and at least an original 73 Muncie had adapters on the trans. The 75 L82 ST10 trans with 355 gears did not have one, the 75 L48 with ST10 and 336's did have one. I have it in my hand in fact since it's been on my desk for a couple of years now.

Here are some pictures I took of it last year

 

[Mac]

You might have something worth holding onto, Gary. My 73 has (had?) one of these adapters. I've still got it but where the thing is pressed together, it keeps popping out so I swapped it for an aftermarket piece. I was thinking of drilling/tapping a set screw or two but I haven't tackled it as yet as I'm worried about cracking the case as it appears to be pot metal.

The problem is the aftermarket replacements are all 1 : 1 ratio instead of 1 : 0.882 which throws the speedo is out of wack. Apparently some of the Camaros had the same adapter as well and the same issue.

-Mac

Hi Ken,
Not sure about the 79's but I know that the 75's we had/have and at least an original 73 Muncie had adapters on the trans. The 75 L82 ST10 trans with 355 gears did not have one, the 75 L48 with ST10 and 336's did have one. I have it in my hand in fact since it's been on my desk for a couple of years now.

Here are some pictures I took of it last year

 

[herminator]

Can't use the calculators effectively as I do not know the actual drive gear (7T or 8T) that was installed in the trans at the factory.

Ordered both 18T & 19T and will go with the one that produces the best results.

Let us know which one worked for you. All of the calculations depend on tire height, gear ratio, etc. I had the same adaptor GTR99 showed pic's of but it spread apart and speedo quit working (like Mac's). When I went with a 1:1 replacement, my speedo was reading too high. Only I was stuck between too high and too low as far as gear teeth on the replcement so I opted for a bit high and went from the OEM 20 to a 21 tooth. Only then did I did find a replacement 1:882 adaptor but it was about $100 and I had already installed the 1:1.

 

[herminator]

Ken79, how did you make out? I was still about 5 MPH over with the 21 tooth so put in a 22 tooth and am right on now-trial and error.

 

[Ken79]

Ken79, how did you make out? I was still about 5 MPH over with the 21 tooth so put in a 22 tooth and am right on now-trial and error.

Have not installed the new gear yet. Will post up when it is done.

 

[Ken79]

Hey Gary - nice chatting with you again this weekend at Carlisle. My 79 4-speed did not come with a trans adapter at all.

I finally replaced the blue 20 tooth with the natural 19 tooth gear. That essentially cut my MPH problem in half. That is, now when my speedo reads 70, I'm actually doing approx 73 per my GPS. So, in my specific case, I now need to replace the 19 tooth with the 18 tooth that I also purchased at the same time, just in case. Will install that at some convenient time in the future.

Ken