Author Topic: Speedo Cable Color (Read 3684 times)

dannystarr · « **on:** August 28, 2023, 09:27:13 PM »

I was told by a friend that I was traveling 75mph. My speedo said 65. He used a satellite application on his cell phone for the speed and pulled up to match my speed. It was EXACTLY 10mph fast. What is the speedometer cable tooth/color. It has a black one I think with 22 teeth in it now.
Transmission is a 4-speed M21 with 3.73 rear axle gears. Tires are Good-Year Wide Tread GT with 3.73 and only 100 miles on them. I see MANY colors and charts online ALL giving different teeth counts with the same color? ... Danny

dannystarr · « **Reply #1 on:** August 30, 2023, 02:00:51 PM »

I forgot the word gear. I don't need the trans gear info. D

camaronut · « **Reply #2 on:** August 30, 2023, 04:40:52 PM »

Danny,

Try this: http://www.novaresource.org/speedo.htm

x66 714 · « **Reply #3 on:** August 30, 2023, 06:44:51 PM »

Danny, according to the factory parts book, it should be black, 23 tooth, pn# 3860346. You speedometer is off by percentages. It just happens to be 10 mph at that speed. No adapter used with that gear ratio....Joe

dannystarr · « **Reply #4 on:** August 30, 2023, 08:25:28 PM »

WOW.. I would have thought after 80 or 100 MPH it would be off. That's the way those old cars worked. But reading 65 but actual speed is 75, that sucks. Any trick to make it more accurate between 50 and 75? Less teeth or more teeth?
Using that tire chart site 373 X 20.2 = 7,534.6 Divided X 26.73 = 28. Only 17 to 24 tooth gears on the chart... SSSOOO? No 28 or 3.73 on that chart... Just thought maybe someone knew that a certain gear with my set-up is more accurate. D

x66 714 · « **Reply #5 on:** August 30, 2023, 08:57:17 PM »

We have speedometer calibration shops here. I would think you would also. Not really knowing how the speedometer works, could the problem be there?...Joe

rare396bronze · « **Reply #6 on:** August 30, 2023, 11:59:33 PM »

When i worked in gm parts the rule of thumb was to add teeth to slow down speedometer. To speed up speedometer subtract teeth. Where to 1 tooth for 5mph. So try 3987921 it is 21 teeth it might be still be available from GM. Also see them on Ebay. 3987920 is 20 teeth . Hope this helps.

dannystarr · « **Reply #7 on:** August 31, 2023, 02:32:52 PM »

As far as I know the speedo is functioning properly. I installed a new cable a while back. Since I bought the car I have put about 5,000 miles on it. And the odometer shows that....

Thank you for the gear info, that is EXACTLY the info I was hoping for. I will start my gear experiment soon. D

GMAD_Van Nuys · « **Reply #8 on:** August 31, 2023, 04:15:37 PM »

You might consider getting a speedometer reducer as they are easy to install:

http://www.camaros.org/forum/index.php?topic=15107.0

dannystarr · « **Reply #9 on:** September 03, 2023, 02:18:43 PM »

Well, I am looking at that speedo gear number and it says it is for an automatic? Does it matter?... Danny

GM ACDelco 3987921 Original Equipment Automatic Transmission Speedometer Driven Gear

David K · « **Reply #10 on:** September 03, 2023, 03:56:00 PM »

You could also drive slower…..75 is the new 65.

x66 714 · « **Reply #11 on:** September 03, 2023, 04:43:35 PM »

Quote from: dannystarr on September 03, 2023, 02:18:43 PM

Well, I am looking at that speedo gear number and it says it is for an automatic? Does it matter?... Danny

GM ACDelco 3987921 Original Equipment Automatic Transmission Speedometer Driven Gear

Depends on what part of the catalog you're looking at. Here's the 4 speed section....Joe
http://www.gmpartswiki.com/getpage?pageid=12660

rare396bronze · « **Reply #12 on:** September 03, 2023, 06:20:34 PM »

Was used on 3'4 speed and automatic!

Mike S · « **Reply #13 on:** September 04, 2023, 03:17:07 AM »

I bought one of these kits for my Muncie and found the right gear color (I forget which one though):
https://www.ebay.com/itm/134008901969?var=0&mkevt=1&mkcid=1&mkrid=711-53200-19255-0&campid=5338590836&toolid=10044&customid=5f9fc9133d5a1ee9c046ade676c27d66

Mike

David K · « **Reply #14 on:** September 04, 2023, 03:31:47 PM »

Maybe?
14 or 15” wheels?

David K · « **Reply #15 on:** September 04, 2023, 03:34:51 PM »

Dunno…can’t hurt.

dannystarr · « **Reply #16 on:** September 30, 2023, 05:35:56 PM »

Well, I got it figured out. It had a 23 that was WAY off. I tried a 22 and it fixed it by 5MPH. Then I tried a 21 and an 18 and it seemed to go the wrong way. Then 20 tooth and BINGO, DEAD ON. It read perfect from 25 to 65. I was amazed how accurate it was. At 70MPH on the speedo, it read by satellite at 72. That was the only real change. And I won't be going that fast or above that speed. I was mainly concerned from 40 to 60. So if you have a M21 - 3:73 and are running Good Year E70/15's, this would be a good starting point for accuracy. Going to check some of my other cars and see what happens. I am shocked GM missed it by that much. But maybe some things changed in thousands of miles? The internal gear was in nice shape. And the 23 tooth I took out was also perfect? Danny

169INDY · « **Reply #17 on:** October 01, 2023, 01:02:37 AM »

Great News "D".

Blue Right? (Gear)
Good info

David K · « **Reply #18 on:** October 01, 2023, 02:53:26 AM »

Glad you got it dialed in. I'm curious how you think GM got something wrong.

rare396bronze · « **Reply #19 on:** October 01, 2023, 05:09:48 AM »

The drive gear might had been changed inside the transmission sometime. The might had put the wrong tooth count gear back in! The black gear should been right. I just told you abought the other gears so you didn't have to take off the tailshaft. Glad it worked.

dannystarr · « **Reply #20 on:** October 02, 2023, 02:25:58 AM »

Indy - Yes, Blue Gear....

DK - If it has the factory set-up GM should have made sure it was accurate. Contact Muncie and work it out.

Bronze - When the trans was pulled for inspection and clutch install, it looked to have never been opened up. The side cover
gasket was original. Everything inside was in good original shape. Maybe they pulled it apart at the tail shaft? I believe I am the
3rd owner and the car sat for almost 30 years. I am going to guess it was never apart. Who knows, I am curious to see what
happens with some other cars. Maybe I'll post my findings. Just glad it is very accurate now. Thanx for the suggestion on gear 6-
pack. ... Danny

News:

Author Topic: Speedo Cable Color (Read 3684 times)