Author Topic: Speedo Cable Color  (Read 1760 times)

dannystarr

  • Hero Member
  • *****
  • Posts: 1745
    • View Profile
Speedo Cable Color
« 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

  • Hero Member
  • *****
  • Posts: 1745
    • View Profile
Re: Speedo Cable Gear Color
« 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

  • Hero Member
  • *****
  • Posts: 676
    • View Profile
Re: Speedo Cable Color
« Reply #2 on: August 30, 2023, 04:40:52 PM »

x66 714

  • Hero Member
  • *****
  • Posts: 1777
    • View Profile
Re: Speedo Cable Color
« 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
See America's First, Chevrolet

1968 Z/28 Corvette Bronze. Black Hounds Tooth. 02E Los Angeles born 3/13/1968 pnt OO. Purchased March 1976
1969 SS396 Yellow/Yellow 08E Norwood born 8/28/1969 pnt 76E. Purchased April 1981

dannystarr

  • Hero Member
  • *****
  • Posts: 1745
    • View Profile
Re: Speedo Cable Color
« 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

  • Hero Member
  • *****
  • Posts: 1777
    • View Profile
Re: Speedo Cable Color
« 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
See America's First, Chevrolet

1968 Z/28 Corvette Bronze. Black Hounds Tooth. 02E Los Angeles born 3/13/1968 pnt OO. Purchased March 1976
1969 SS396 Yellow/Yellow 08E Norwood born 8/28/1969 pnt 76E. Purchased April 1981

rare396bronze

  • Member
  • ***
  • Posts: 278
    • View Profile
Re: Speedo Cable Color
« 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

  • Hero Member
  • *****
  • Posts: 1745
    • View Profile
Re: Speedo Cable Color
« 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

  • Member
  • ***
  • Posts: 291
    • View Profile
Re: Speedo Cable Color
« 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

  • Hero Member
  • *****
  • Posts: 1745
    • View Profile
Re: Speedo Cable Color
« 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

  • Hero Member
  • *****
  • Posts: 1119
  • Dr. Longblock
    • View Profile
Re: Speedo Cable Color
« Reply #10 on: September 03, 2023, 03:56:00 PM »
You could also drive slower…..75 is the new 65.

x66 714

  • Hero Member
  • *****
  • Posts: 1777
    • View Profile
Re: Speedo Cable Color
« Reply #11 on: September 03, 2023, 04:43:35 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

See America's First, Chevrolet

1968 Z/28 Corvette Bronze. Black Hounds Tooth. 02E Los Angeles born 3/13/1968 pnt OO. Purchased March 1976
1969 SS396 Yellow/Yellow 08E Norwood born 8/28/1969 pnt 76E. Purchased April 1981

rare396bronze

  • Member
  • ***
  • Posts: 278
    • View Profile
Re: Speedo Cable Color
« Reply #12 on: September 03, 2023, 06:20:34 PM »
Was used on 3'4 speed and automatic!

Mike S

  • Hero Member
  • *****
  • Posts: 2660
    • View Profile
Re: Speedo Cable Color
« 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
67 04B LOS SS/RS L35 Hardtop - Original w/UOIT
67 05B NOR SS/RS L35 Convertible - Restored

David K

  • Hero Member
  • *****
  • Posts: 1119
  • Dr. Longblock
    • View Profile
Re: Speedo Cable Color
« Reply #14 on: September 04, 2023, 03:31:47 PM »
Maybe?
14 or 15” wheels?

David K

  • Hero Member
  • *****
  • Posts: 1119
  • Dr. Longblock
    • View Profile
Re: Speedo Cable Color
« Reply #15 on: September 04, 2023, 03:34:51 PM »
Dunno…can’t hurt.

dannystarr

  • Hero Member
  • *****
  • Posts: 1745
    • View Profile
Re: Speedo Cable Color
« 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

  • Hero Member
  • *****
  • Posts: 1768
    • View Profile
Re: Speedo Cable Color
« Reply #17 on: October 01, 2023, 01:02:37 AM »
Great News "D".

Blue Right? (Gear)
Good info
Jim
68 SS/RS L35 Th-400 LOS
69 Pace Car L48 Th-350 LOS
68 Z28 M21 LOS

David K

  • Hero Member
  • *****
  • Posts: 1119
  • Dr. Longblock
    • View Profile
Re: Speedo Cable Color
« 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

  • Member
  • ***
  • Posts: 278
    • View Profile
Re: Speedo Cable Color
« 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

  • Hero Member
  • *****
  • Posts: 1745
    • View Profile
Re: Speedo Cable Color
« 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