1967 C2 odometer not working and speedometer off

[Robert K.]

Amother problem on my newly acquired 67 C2. The speedometer works, but registers about 10 mph slow. The odometer does not work at all - same with the trip odometer. Is there just one cable for the speedometer and the odometer, or does each have its own. Would appreiciate any help on where to start. Thanks in advance

and thanks again for helping solve the brake light issue.

[Gary B.]

[Mike T.]

Re: 1967 C2 odometer not working and speedometer off

Robert - I've used Joe Ray in Arkansas on 4 different clusters and was always happy with his work.
As to the speedo being off, as Gary mentioned, there's a good chance it's the easily replaceable 'plastic driven gear'
at the trans-end of the speedo cable. Here's a thread I posted on over on the Corvette Forum that has some pics and
a better explanation as to how to access that driven gear.
C2 Lubricating speedo cable from the bottom end - CorvetteForum - Chevrolet Corvette Forum Discussion And, when you do get the old driven gear out, it will be
a certain color and tooth count. My ballpark guess/starting point for a replacement driven gear would be one that is
2-teeth 'less' than your current one. That should help speed up the speedo indication.
Good luck.

[Robert K.]

Re: 1967 C2 odometer not working and speedometer off

Thank you - the forum has been great! I read the stamping on the differential and it corresponds to a 3:08. I will order the replacement drive gear. Suggestions for ordering parts.

Thank you

[Mike T.]

Re: 1967 C2 odometer not working and speedometer off

Robert - pretty sure all the regular Corvette Parts Vendors, like Corvette Central and Zip, should have those driven gears.
You'll also find them available on eBay and Amazon. Most times, it is possible to determine the driven gear needed by
the stock application, meaning rear gear ratio, tire size, etc but the fact that yours is roughly 10 mph off, that might indicate
someone has changed things over the years so...my suggestion would be to hold off on ordering until you get the existing driven
gear out and can verify the color/number of teeth it has and then order a replacement. Keep in mind, that it's not an exact science
because one tooth isn't always exactly a 5mph change.

[Duke W.]

Re: 1967 C2 odometer not working and speedometer off

There are two kinds of speedometer error, offset and gain.

Offset error is essentially the same at all speeds. Gain error is a nearly constant PERCENT of speed.

In order to determine which one you have or some combination of both you need to report error at several different speeds, say from 20 MPH to as high a speed as you consider safe to test.

So in order to make a rational diagnosis, you need to provide more data including the installed tire size and revs/mile.

A 3.08 axle with OE size tires uses an 18 tooth driven gear. It's easy to remove and count teeth, so do that along with the reporting color and the speedometer reading vs. actual speed at as many speeds as you can in 5 to 10 MPH increments.

ONLY then can ANYONE make a rational diagnosis for the speedomenter error. The odometer is driven off the speedometer, so a a repair would require removing the IP and sending the speedometer to a specialist for repair, but try to reset it several times first.

Duke

[Robert K.]

Re: 1967 C2 odometer not working and speedometer off

Great advice. I will remove the gear and count teeth and see color.

Thank you -
Bob

[Mark F.]

Re: 1967 C2 odometer not working and speedometer off

Also, there are people (like myself) who have rebuilt there diff w/ a different ratio in the guts than what the stamp says it should be on the case...

all of what everyone has said above affects accurate speedo response...

after rebuilding my diff; putting the right trans plastic gear in there; and checking tire sizes; I got on a straight stretch of interstate and timed myself on numerous (alternating) 1-mile marker posts...

it was close enough for jazz AFAIWC...

[Stephen L.]

Re: 1967 C2 odometer not working and speedometer off

Easy to check the actual differential ratio:
Jack up BOTH wheels. Rotate the wheels to obtain 1 rotaion of the wheels. Then note how many revs the input driveshaft made. Example: 1 wheel rotation will show just over 3 driveshaft rotations for a 3.08 ratio. For a more accurate measurement, rotate the wheels 10 times while counting the driveshaft rotations. Then divide by 10 for the ratio.......

[Robert K.]

Re: 1967 C2 odometer not working and speedometer off

Thank you for all the responses. I love my - new to me- 67 and am learning fast.

- Bob

[Mike T.]

Re: 1967 C2 odometer not working and speedometer off

Robert - having a speedo be a little 'off' is just one of the many things we sometimes have to deal with on these old cars and
I'm sure you'll get it squared away soon. By the way, love to see some pics of your new 67 and if you've already
posted some on another thread, let us know which one.
Good luck.

[Owen L.]

Re: 1967 C2 odometer not working and speedometer off

Robert - having a speedo be a little 'off' is just one of the many things we sometimes have to deal with on these old cars ...

Yup. I often pull up the speedo app on my phone to keep an eye on my true speed.

[Mike T.]

Re: 1967 C2 odometer not working and speedometer off

Same here, Owen. That app works well for double checking true speed. IMG_2165.jpg