[C1] Speedometer rebuld time?
Thread Starter
Melting Slicks
Joined: Jan 2008
Posts: 3,124
Likes: 181
From: Concord CA
Speedometer rebuld time?
My speedometer is reading low...here some measurements
Speedometer......GPS
10...……………...12
20...……………...35
40...……………...60
Seems to be off about at an average of about 17mpg...that's a lot.
Here is what I figured out.
Tires are 27.1
Turning the wheel one full rotation; best I can tell, the drive shaft rotates 1.60 times; so I figure this is a 3.36 ratio. (I stated another POST asking if anyone knows what the differential is).
I have the white (19-tooth) speed gear in my 4 speed Muncie. I'm not sure if this is correct.
Next I need to spin the speedometer cable at 1,000rpm's and see if the speedometer is reading 60mph. I have a tool that can measure this.
Speedometer......GPS
10...……………...12
20...……………...35
40...……………...60
Seems to be off about at an average of about 17mpg...that's a lot.
Here is what I figured out.
Tires are 27.1
Turning the wheel one full rotation; best I can tell, the drive shaft rotates 1.60 times; so I figure this is a 3.36 ratio. (I stated another POST asking if anyone knows what the differential is).
I have the white (19-tooth) speed gear in my 4 speed Muncie. I'm not sure if this is correct.
Next I need to spin the speedometer cable at 1,000rpm's and see if the speedometer is reading 60mph. I have a tool that can measure this.
Last edited by jusplainwacky; Jun 26, 2019 at 01:11 AM.
Race Director
Joined: May 2000
Posts: 12,712
Likes: 2,271
Manual trans or Powerglide?
You have to look at the percent difference at each speed, and it's all over the place, so a simple driven gear change will not have much effect. OE OD tires and a 3.55 axle usually calls for a 21 tooth driven gear with a manual transmission. I haven't heard of a white 19T gear so I wonder if it's the correct diameter family for the drive gear inside the transmission. and it may be slipping.
Another issue might the that the speedometer cable needs to be cleaned and lubricated, but get the proper driven gear installed first. When it's out record what color the drive gear is and that should get you a tooth count with the proper reference such as the Corvette parts catalog, but there might be a Web site that has the proper part numbers for OE diameter tires and a 3.55 axle.
Duke
You have to look at the percent difference at each speed, and it's all over the place, so a simple driven gear change will not have much effect. OE OD tires and a 3.55 axle usually calls for a 21 tooth driven gear with a manual transmission. I haven't heard of a white 19T gear so I wonder if it's the correct diameter family for the drive gear inside the transmission. and it may be slipping.
Another issue might the that the speedometer cable needs to be cleaned and lubricated, but get the proper driven gear installed first. When it's out record what color the drive gear is and that should get you a tooth count with the proper reference such as the Corvette parts catalog, but there might be a Web site that has the proper part numbers for OE diameter tires and a 3.55 axle.
Duke
Last edited by SWCDuke; Jun 25, 2019 at 08:05 PM.
Thread Starter
Melting Slicks
Joined: Jan 2008
Posts: 3,124
Likes: 181
From: Concord CA
Perhaps it's not white but neutral color and it has 19-teeth.
The speedometer reading (needle) is solid and not jumping around. It has been cleaned and lubricated with graphite spray.
From what I have read, the thing to do is to get the right gear so the odometer reads correctly, then figure out the speedometer reading.
Like you, I also came to discover it should be a red (21-teeth) gear, but from what I understand, this will make the speedometer read even slower.
So, it's looking like the speedometer needs adjusting.
The speedometer reading (needle) is solid and not jumping around. It has been cleaned and lubricated with graphite spray.
From what I have read, the thing to do is to get the right gear so the odometer reads correctly, then figure out the speedometer reading.
Like you, I also came to discover it should be a red (21-teeth) gear, but from what I understand, this will make the speedometer read even slower.
So, it's looking like the speedometer needs adjusting.
