Speedometer drive gear

[Steve H.]

Hello all,
Does anyone know the number of teeth that the spedometer drive gear (the plastic gear in the transmission) should have? The 67' has a 327 350 hp with 4 speed CR and 3.70 rear. The tires are correct size.
While I am at it does anyone know the same for a 56' with 265 with 3 speed and 3.70 rear?

Thanks,

Steve

[Joe L.]

Re: Speedometer drive gear

Hello all,
Does anyone know the number of teeth that the spedometer drive gear (the plastic gear in the transmission) should have? The 67' has a 327 350 hp with 4 speed CR and 3.70 rear. The tires are correct size.
While I am at it does anyone know the same for a 56' with 265 with 3 speed and 3.70 rear?

Thanks,

Steve

Steve------


The DRIVE gear had 8 teeth. However, there are several gears with 8 teeth and there are two different OD's for these gears.

As a matter of fact, you need the exact same DRIVE gear for BOTH of the applications you mentioned. This gear was GM #3708145 but it's been discontinued for quite awhile. I believe it's available in the aftermarket from speedometer specialists. It's a STEEL gear of 1.77" OD.

[Wayne M.]

Re: Speedometer drive gear

On the oft-chance that you're referring to the plastic (or nylon) DRIVEN gear and not the steel DRIVE gear, then your '67 needs the green 22-tooth gear #3860345 (shown on left). And if still orignal, your trans output shaft will have the steel gear that Joe L. mentions.

As for 1956, my 1970 catalog says (also) 22-tooth nylon solid green # 3708699 for '56-60 w/3.70 axle. Don't know the details, but this gear might have a longer shaft. Probably no longer available.



m

[Steve H.]

Re: Speedometer drive gear

Joe and Wayne,
Thanks for the information. I was refering to the plastic gear in the transmission that drives the cable.

Steve

[Richard G.]

Re: Speedometer drive gear

My Speedometer has to be off. According to the above post engine rpm at 60mph should be 2800 rpm. I was over 3200 at an indicated 60.
Believer the differential was changed from the 4:11 to the 3:55 and the Speedometer gear wasn’t corrected.
Should be a 14.6% decrease of rpm at the same speed. Sure looks like the gear change is at the root of my speed issue.

I have included some pictures of my driven gear setup out of my 1963 with a Borg-Warner.
Also included pictures of Speedometer drive gears out of a 1959 Borg-Warner.

I removed a 24 tooth yellow plastic gear from the transmission. This was the largest diameter gear that will fit as I had to work it through the hole to remove it. I suspect this was the original gear for the 4:11 gears.

I pulled the 1959 just for comparison purposes. The 63 parts are on the left.


The 63's retaining bolt has a large "R" on the head. This is the first one of these I have found on my 63. Also is a dimple and a corner cut out of the bullet retainer.


This is the only marking I could find on the plastic gear. I know later models have the tooth count embossed on them. Appears to be a 9S.


However I am still confused.
The difference between the 22T (as found) and the 24 tooth gears (recommended) is 8.7%
The difference between the 4:11 and the 3:55 is 14.6%

If I used a 21T gear the difference would be 13.3%.

Seems this would be a better choice if the gear in the transmission is indeed original to the 4:11's.
What I didn't do it confirm my speed, stupid I know.
I may have to put the old gear back in and get some accurate speed information to make this a one time change.
Rick

[Richard G.]

I think the archives answered my question; I found this after writing the post;

Keith, the plastic gear on the tailshaft of the trans is probably your problem. A '65 with the original 4.11 drivetrain should be using the yellow driven gear (24 teeth). I'm guessing that your "white" gear is what is referred to in the parts manual as "natural" with 19 teeth, which is halfway between the 18 tooth brown gear (for a 3.08 ratio) and 20 tooth blue (for a 3.36 ratio). These gears, as well as the 21 tooth red gear (for 3.55 ratios) all require the speedometer DRIVE gear # 3920498 (on the output shaft of the trans).

[Joe L.]

Re: Speedometer drive gear

Rich------

If you removed a 24 tooth gear, this means that you have the "small OD" DRIVE gear installed on the transmission mainshaft. The smallest tooth count DRIVEN gear that you can use with this DRIVE gear is a 22 tooth GREEN gear. No lesser tooth count DRIVEN gear can be used. You have two options to correct your speedo:

1) you can partially disassemble the transmission (out of the car) and change the DRIVE gear;

2) install a speedometer adapter to correct. An adapter is a small gearbox that changes the output speed of the DRIVEN gear.

Your speedometer fitting ("bullet") and retainer appear correct for 1963. The 1963+ retainers typically have the "clipped corner" configuration.

As you show, most C1 speedometer fittings and DRIVEN gears are the longer style.

Use a portable GPS unit to verify your actual speedometer error.

[Richard G.]

Re: Speedometer drive gear

Let me recap my understanding of this system.

1) There are two possible pressed on steel gears that are installed on the transmission output shaft.
2) They have two different outside diameters but the same gear count.

3) My 24 tooth and a 22T plastic gears require the small diameter steel gear.
4) I require a 21T gear to correct the speedometer.
5) This 21T plastic gear requires a larger diameter steel gear on the output shaft.
6) To compensate for the smaller diameter plastic gear the steel gear was made larger so they would mess correctly.
7) I would need the larger steel gear and would have to pull the transmission to change out the gear. (From small to large)
8) After the transmission is out, I would only need to pull the tail shaft housing, including reverse gear to change out the steel gear.

I have both straight and 90 deg versions of the speedometer adapters here. I could correct the speedometer using only the geared box, with the correct ratio installed. However, It isn't likely to work well because of the speedometer cable routing restrictions.

I had last driven this care some 37 years ago.
As soon as I drove the Corvette and saw the speedometer readings I remembered the speedometer was off.
I never knew why till I decoded the differential during the rebuild.
I wish I had remembered this before I installed the body.

I have some steel gears here also. Some from Power Glide transmissions and others from 4 Speeds.
I guess I need to know the outside diameters of each gear to identify which ones I have (large or small diameter)?
I am told the steel gear has 8 teeth.
Rick

[Joe L.]

Re: Speedometer drive gear

Let me recap my understanding of this system.

1) There are two possible pressed on steel gears that are installed on the transmission output shaft.
2) They have two different outside diameters but the same gear count.

3) My 24 tooth and a 22T plastic gears require the small diameter steel gear.
4) I require a 21T gear to correct the speedometer.
5) This 21T plastic gear requires a larger diameter steel gear on the output shaft.
6) To compensate for the smaller diameter plastic gear the steel gear was made larger so they would mess correctly.
7) I would need the larger steel gear and would have to pull the transmission to change out the gear. (From small to large)
8) After the transmission is out, I would only need to pull the tail shaft housing, including reverse gear to change out the steel gear.

I have both straight and 90 deg versions of the speedometer adapters here. I could correct the speedometer using only the geared box, with the correct ratio installed. However, It isn't likely to work well because of the speedometer cable routing restrictions.

I had last driven this care some 37 years ago.
As soon as I drove the Corvette and saw the speedometer readings I remembered the speedometer was off.
I never knew why till I decoded the differential during the rebuild.
I wish I had remembered this before I installed the body.

I have some steel gears here also. Some from Power Glide transmissions and others from 4 Speeds.
I guess I need to know the outside diameters of each gear to identify which ones I have (large or small diameter)?
I am told the steel gear has 8 teeth.
Rick

Rick------

Your 1 through 8 statements are correct.

Your statement regarding the adapters is also correct.

I'm 7,000 miles away from my references so I can't check the DRIVE gear diameters. As I vaguely recall, the small one might be 1.77" OD and the large one 1.84" OD but it's only a vague recollection. There are only 2 common DRIVE gears that were used through the C1 and C2 years. One has a GM part number ending in 44 and the other ending with 45. There were also a few other "oddball" gears used for very low and very high rear gear ratios (like 2.73:1 or 4.88:1) but one hardly ever runs into those.

[Peter M.]

Re: Speedometer drive gear

Rick,
Joe's memory is incredible!
3708144 drive gear .609" thickness X 1.85" O.D.
3708145 .531" X 1.75" O.D.
I find it easier to tell them apart by the thickness.
Kind Regards

IN APPRECIATION OF JOE LUCIA

[Richard G.]

Re: Speedometer drive gear

Thanks to everyone for the help.
I just don't know where one would get this kind of information except here!
I only have to find my gear stash and wait till winter.
Highly likely I have the one I need as i expect the correct gear for the 4:11 ratio is the difficult one to find.
I now have all the information I need.
Thanks Joe for the help from half way around the world.
Now if I can just figure out which shoe goes on which foot!
Take Care.
Rick