C2 speedometer driven gear help

[Donald H.]

According to my GPS my 66 speedometer is reading about 15MPH low. Here is what I have:

The differential housing is stamped AM and dated 10 22 65. I don't know if it is original to the car. The car's birth date is July 66. The AM should translate to 3:36 posi. I will do the wheel / drive shaft rotation test when I have a chance to get under the car.

4 speed tranny and the tires are 205/75R15

I did a couple of checks against my GPS:

at 40MPH reading on the speedometer the GPS shows 56MPH.

at 2500RPM on the Tach, the GPS shows 61MPH. I used the Keisler website and its speed analyzer and it showed at 2500RPM and a Muncie in 4th gear with a 3:36 rear should be 60MPH. So maybe my speedometer is simply not calibrated correctly.

The driven gear in the transmission is blue, which I believe is 20 tooth. And I also believe this is correct for 3:36 rear. According to what I have read, adding a tooth should increase the speedometer 5MPH, so If I install a 23 tooth driven gear it should get my speedometer much closer to actual speed.

Does going to a Black 23 tooth driven gear sound Right?

Or, does it look like my Speedometer is just not calibrated correctly?

Thanks,

Don

[John H.]

Re: C2 speedometer driven gear help

The driven gear in the transmission is blue, which I believe is 20 tooth. And I also believe this is correct for 3:36 rear.
Or, does it look like my Speedometer is just not calibrated correctly?

Thanks,

Don

Don -

Verify the installed axle ratio doing the turn-the-wheels trick and see what it tells you.

My '67 has a 3.36 "AM" axle and 205/75-15 radials on stock rims with a 20-tooth blue driven gear, and the (restored/calibrated) speedo is dead-on accurate at 70 mph.

[Chris E.]

Re: C2 speedometer driven gear help

If the speedo is reading LOW, you need to REDUCE the tooth count.

http://www.stl-vettes.com/65Vette/GearCalc.htm

[Donald H.]

Re: C2 speedometer driven gear help

If the speedo is reading LOW, you need to REDUCE the tooth count.

http://www.stl-vettes.com/65Vette/GearCalc.htm

Right, I had it reversed above. I need fewer teeth on the driven gear so it spins faster.

[Donald H.]

Re: C2 speedometer driven gear help

OK, so I did the wheel / drive shaft rotation test. One full turn of the tire (both tires off the ground and posi), and the drive shaft did 3 revolutions plus a little over another 1/4 revolution. So I feel comfortable it is a 3:36 rear.

So if I have the correct (blue 20 tooth) driven gear for a 3:36 and my speedometer is reading 15 MPH low, then I guess my speedometer is in need of some work.

Would it be worthwhile to go ahead and put in a Brown (18 tooth) driven gear to get it closer. I don't really want to tear the dash apart to get the speedometer out right how, but I also don't like that the speedometer is so far off.

Thanks,

Don

[Bob R.]

Re: C2 speedometer driven gear help

I have tried to count the revolutions to determin the ratio and it is not as easy as it seems. There isn't that much difference between a little more then 3 1/4 revolutions and 3 1/2 revolutions. My 63 rear is stamped as a 3:08 but at some point it was rebuilt and changed to a 3:36. With the blue gear mine speedometer is quite accurate. It is a lot of work to remove the cluster and have it calibrated I thiink I would go with your thought and try a different gear.

[Donald H.]

Re: C2 speedometer driven gear help

I have tried to count the revolutions to determin the ratio and it is not as easy as it seems. There isn't that much difference between a little more then 3 1/4 revolutions and 3 1/2 revolutions. .

I'm pretty certain now that I have a 3:36. I actully put index marks on the wheel and the drive shaft.

[Michael W.]

Re: C2 speedometer driven gear help

So if I have the correct (blue 20 tooth) driven gear for a 3:36 and my speedometer is reading 15 MPH low, then I guess my speedometer is in need of some work.

Would it be worthwhile to go ahead and put in a Brown (18 tooth) driven gear to get it closer. I don't really want to tear the dash apart to get the speedometer out right how, but I also don't like that the speedometer is so far off.

Thanks,

Don

To confirm that it's the speedo mechanism that's off vs. something weird going on with the gear ratios, go for a drive over a measured distance and see if the odometer is accurate or not.

The result will guide you as to how best to remedy the issue.

[Duke W.]

Re: C2 speedometer driven gear help

You have to determine if you have an offset error, a gain error or some conbination of the two.

An offset error means that the speedo has a constant error in MPH regardless of speed. A gain error is an error that is a constant percent at any speed.

Tests at mulitple speeds is the data you need to make this determination. You need more test data. Check GPS readings a 30, 40, 50, 60, and 70 MPH... higher if an opportunity arises.

Offset error can be corrected by pulling the needle off the speedo and clocking it an appropriate amount. This should be done by an experienced speedometer tech with the proper equipment.

Gain error can be corrected by using an appropriate speedometer driven gear or external adapter.

Duke

[Donald H.]

Re: C2 speedometer driven gear help

You have to determine if you have an offset error, a gain error or some conbination of the two.

An offset error means that the speedo has a constant error in MPH regardless of speed. A gain error is an error that is a constant percent at any speed.

Tests at mulitple speeds is the data you need to make this determination. You need more test data. Check GPS readings a 30, 40, 50, 60, and 70 MPH... higher if an opportunity arises.

Offset error can be corrected by pulling the needle off the speedo and clocking it an appropriate amount. This should be done by an experienced speedometer tech with the proper equipment.

Gain error can be corrected by using an appropriate speedometer driven gear or external adapter.

Duke

Thanks Duke,

I'll do some more testing.

[Joe L.]

Re: C2 speedometer driven gear help

Thanks Duke,

I'll do some more testing.

Donald------


The next thing you need to do is to confirm what speedometer driven gear you have installed now. That's very easy to do.

[Donald H.]

Re: C2 speedometer driven gear help

Donald------


The next thing you need to do is to confirm what speedometer driven gear you have installed now. That's very easy to do.

I did mention above that it is blue. I didn't pull it out but can see the end of it with the speedometer cable off

[Donald H.]

Re: C2 speedometer driven gear help

You have to determine if you have an offset error, a gain error or some conbination of the two.

An offset error means that the speedo has a constant error in MPH regardless of speed. A gain error is an error that is a constant percent at any speed.

Tests at mulitple speeds is the data you need to make this determination. You need more test data. Check GPS readings a 30, 40, 50, 60, and 70 MPH... higher if an opportunity arises.

Offset error can be corrected by pulling the needle off the speedo and clocking it an appropriate amount. This should be done by an experienced speedometer tech with the proper equipment.

Gain error can be corrected by using an appropriate speedometer driven gear or external adapter.

Duke

Duke, here are additional test results:

Speedo GPS Difference Tach
22 30 8 1100
29 40 11 1550
36 50 14 2000
43 60 17 2400
51 70 19 2850

So this indicates a gain error. But the difference seems to be greater than I can get by changing the driven gear. I currently have a blue gear (20 tooth) and even if I go down to the lowest 18 tooth it will not make up the difference.

[Duke W.]

Re: C2 speedometer driven gear help

Yes, it appears to be gain error - computes in the range of 36 to 40 percent which is fairly consistent given the accuracy of GPS and how close you can read the speedo.

Your revs at 60 indicate the car has a 3.08 axle, but the 18T driven gear will only give about 10 percent correction to the current situation.

The only thing I can think of right now is that you have an incorrect internal drive gear. Post the color and try to count the teeth if you can.

Is the transmission original to the car? If so you should be able to see the VIN derivative stamp with the car up in the air. You should also do a manual count of driveshaft revs per halfshaft rev to verify the installed axle ratio.

Duke

[Donald H.]

Re: C2 speedometer driven gear help

Yes, it appears to be gain error - computes in the range of 36 to 40 percent which is fairly consistent given the accuracy of GPS and how close you can read the speedo.

Your revs at 60 indicate the car has a 3.08 axle, but the 18T driven gear will only give about 10 percent correction to the current situation.

The only thing I can think of right now is that you have an incorrect internal drive gear. Post the color and try to count the teeth if you can.

Is the transmission original to the car? If so you should be able to see the VIN derivative stamp with the car up in the air. You should also do a manual count of driveshaft revs per halfshaft rev to verify the installed axle ratio.

Duke

The rear is stamped AM and for 66 that should be 3:36. I was pretty careful counting the drive shaft turns at one full wheel turn. Indexed both the wheel and drive shaft to make sure I could accurate coutn and I really think I have a 3:36. So, maybe there is something else off.

Yes the VIN is repeated on the engine pad and transmission, so the transmission should be original. Of course, I guess someone could have changed the drive gear. I have not taken the driven gear out, but when I pulled the speedo cable off I could clearly see the end which was Blue.

I didn't think the driven gears would even work with the wrong drive gear?

Is there any way to figure out which drive gear in in the tail without pulling the transmission apart? Would a test using a driven gear for the taller gears be worth while?

Thanks,

Don