C3 - Need help with Speedo Adapter

[Christopher K.]

Decided to fix speedometer on my '73 L82 4spd. It has a STOCK right angle adapter. It's busted. Speedo cable tests good, speedo driven gear good.

Where can I get this fixed or replaced? I want to keep this car stock.

Second question is the GM part # on the adapter. It is 6478879, ratio
.882 which is listed in AIM (12j, 6.5) for a 4.11 rear axle. I believe this was only available with M21 and Z07 package. I haven't been able to read the code on the rear end yet, the flange is covered in undercoating. I don't have a tank sticker and haven't found the build sheet yet.

My transmission is an M20, matching vin #. Speedo driven gear is natural 19 teeth as listed for a 3.70 rear in AIM 12j, 6, but it lists a different adapter, part #6477965.

Is there a chance GM just put on the same adapter for both since the
4.11 rear has a silver 22 teeth driven gear?

Maybe someone has a similar optioned car. Thanks for any info, this is really a puzzle! Chris

[Robert S.]

Re: C3 - Need help with Speedo Adapter

Here is some info - not sure if it will help your problem or not. I chased an intermittant Speedometer problem on my son's 74' L82 speed for quite a while. In the end the problem turned out to be slight wear in the groove on the speedo 90 degree adapter where the small ,flat attachment plate on the gearbox mates. The wear, which was barely perceptible, was enough to allow the 90 degree adapter driven gear to move out of contact with the gear within the gearbox and hence the speedo would stop working.
The solution was to make a new plate out of slighlty thicker steel and to sand the plate down, little by little ,until it had a very snug fit in the groove on the 90 degree adapter. Reinstalled everything and the speedometer has been working fine now for many years.
P.S. That discovery was the end of 1.5 years of chasing the intermittant problem.

[Christopher K.]

Re: C3 - Need help with Speedo Adapter

Thanks Robert, that's a good idea and great job solving that mystery!

Unfortunately, I think it's something internal on the adapter in my case. If I spin the input end (gear end) the output end does not rotate. I assume yours must have worked when you tested it this way.

Appreciate the idea though, Chris

[Verle R.]

Re: C3 - Need help with Speedo Adapter

"Decided to fix speedometer on my '73 L82 4spd. It has a STOCK right angle adapter. It's busted. Speedo cable tests good, speedo driven gear good.

Where can I get this fixed or replaced? I want to keep this car stock."

Speedometer Repair Service
1502 E 11th St
Tulsa, Ok 74120

918-584-7698

They can repair or replace your right angle adapter.
There were an authorized GM repair center when there was such a thing.
They can repair and calibrate speedometers and tachometers correctly.
They do not do "restoration".

They are fast and reasonable.

Verle

[Harmon C.]

Re: C3 - Need help with Speedo Adapter

Decided to fix speedometer on my '73 L82 4spd. It has a STOCK right angle adapter. It's busted. Speedo cable tests good, speedo driven gear good.

Where can I get this fixed or replaced? I want to keep this car stock.

Second question is the GM part # on the adapter. It is 6478879, ratio
.882 which is listed in AIM (12j, 6.5) for a 4.11 rear axle. I believe this was only available with M21 and Z07 package. I haven't been able to read the code on the rear end yet, the flange is covered in undercoating. I don't have a tank sticker and haven't found the build sheet yet.

My transmission is an M20, matching vin #. Speedo driven gear is natural 19 teeth as listed for a 3.70 rear in AIM 12j, 6, but it lists a different adapter, part #6477965.

Is there a chance GM just put on the same adapter for both since the
4.11 rear has a silver 22 teeth driven gear?

Maybe someone has a similar optioned car. Thanks for any info, this is really a puzzle! Chris

I have seen the .882 adaptor on my 73 four speeds with a 3.36. About five years ago I was able to buy them at the local GM dealer. The shape was different and you had to order two keys to convert them to the square style ends. This has came up before and as I recall truck drive line shop may have adaptors.

[Dick W.]

Re: C3 - Need help with Speedo Adapter

Look up S. S. White Company. They make/did make the adapters. Apparently they purchased all the parts and tooling from SW. Older HD truck dealers may still have their kits and be able to make you one up

[Michael W.]

Re: C3 - Need help with Speedo Adapter

I've got an .882 adapter on my car, same as Lyle. The adapters have a press-in sleeve on the input side that can sometimes work loose allowing the drive pin to disengage. Make sure that the sleeve on your is pushed in and secured properly before condemning it.

[Christopher K.]

Re: C3 - Need help with Speedo Adapter

Thanks for all the replies and vendors guys. It's interesting, Lyle, that they would put this adapter on a 3.36 rear, obviously there were other combos besides those listed in the AIM. In addition to the GM part # mine also says Hubbard on the two chrome end caps.

Michael, I'm not sure about the "sleeve" you refer to. The speedo end seems secure and will rotate if I move or twist it. However, moving the gear end, which does move the Key which fits in the adapter, will move freely but will not move the speedo cable end.

Thanks again for the help, Chris

[Harmon C.]

Joe L. we need some help!

We need Joe with part numbers and if GM still sells them.
Lyle

[Joe L.]

Re: Joe L. we need some help!

We need Joe with part numbers and if GM still sells them.
Lyle

Lyle and Christopher-----


The GM #6478879 adapter was used for most, if not all, 1973 Corvettes WHICH USED AN ADAPTER (not all axle ratios required an adapter). The ratio of the 6478879 was 0.882:1. Different axle ratios were accommodated by different speedometer DRIVEN gears used with or without the adapter.

The 19 tooth NATURAL speedometer DRIVEN gear was used for M-20/21 WITHOUT ADAPTER with 3.08:1 rear ratio and M-20/21 WITH ADAPTER 6478879 for 3.55:1 ratio.

The GM #6478879 was discontinued in October, 1976. There were replacement part numbers but, from what I can tell, all are now discontinued, too. However, a speedometer shop should be able to sell you a functional adapter of the general original configuration. They may even be able to repair the one you have with new internal parts, as required.

I can find no indication that the GM #6477965 adapter was ever originally used on a 1973 Corvette.

[Harmon C.]

Re: C3 - Need help with Speedo Adapter

Thanks Joe
When I find the ones I bought I will get back with a part number.
Lyle

[Christopher K.]

Re: C3 - Need help with Speedo Adapter

Thanks Joe, that's some very clear, concise info.

Just curious, what's your source for that info - St Louis production info, judging manual/experience, or something else?

I appreciate your help, Chris

[Joe L.]

Re: C3 - Need help with Speedo Adapter

Thanks Joe, that's some very clear, concise info.

Just curious, what's your source for that info - St Louis production info, judging manual/experience, or something else?

I appreciate your help, Chris

Chris-----


A combination of a lot of things, not including judging experience.

[Larry M.]

Re: C3 - Need help with Speedo Adapter

Joe,

When I bought our 1972 4-speed, 3.70:1 rear axle, it had a 6477966, 1:1.214 adapter on the transmission, along with a 19-tooth gear. With the adapter in place, the speedometer read much higher than actual speed (on a 55 mph highway, keeping up with the flow of traffic, the speedo would read close to 90 mph), and on a measured mile, the trip odometer would accrue approximately 1.4 miles.

At the time, knowing very little about the car, I thought the adapter was the wrong one for the car. A Corvette shop let me know my car didn't need the adapter, only a different gear, and that someone in the past had likely installed the adapter as a way of mating the speedometer cable to the transmission without the need to make the right-angle turn the cable needs to make.

Minus the adapter, and with a different gear, the car clocks measured miles accurately, and the speedometer reading seems to be consistent with other traffic on highways.

Do you know if that adapter was a Corvette piece, or for other GM vehicles?

Thanks,
Larry

[Joe L.]

Re: C3 - Need help with Speedo Adapter

Joe,

When I bought our 1972 4-speed, 3.70:1 rear axle, it had a 6477966, 1:1.214 adapter on the transmission, along with a 19-tooth gear. With the adapter in place, the speedometer read much higher than actual speed (on a 55 mph highway, keeping up with the flow of traffic, the speedo would read close to 90 mph), and on a measured mile, the trip odometer would accrue approximately 1.4 miles.

At the time, knowing very little about the car, I thought the adapter was the wrong one for the car. A Corvette shop let me know my car didn't need the adapter, only a different gear, and that someone in the past had likely installed the adapter as a way of mating the speedometer cable to the transmission without the need to make the right-angle turn the cable needs to make.

Minus the adapter, and with a different gear, the car clocks measured miles accurately, and the speedometer reading seems to be consistent with other traffic on highways.

Do you know if that adapter was a Corvette piece, or for other GM vehicles?

Thanks,
Larry

Larry-----


Adapters were used on Corvettes as well as other Chevrolets and GM cars. For the most part, they were used on 1971+ models, although some were originally used earlier, too.

The 1971+ Muncie and BW T-10 transmission had only one configuration speedometer DRIVE gear unlike earlier which had two different available gears. The 1971+ speedometer DRIVE gear was only compatible with the 18-22 (silver) tooth DRIVEN gears. So, for rear axle ratio applications which would have earlier required a 23 or higher driven gear tooth count, an adapter was required for 71+.

For 1972 with 3.70 rear ratio, a DRIVEN gear of 22 tooth (silver) should result in an acceptable speedometer calibration without the use of an adapter. The 22 tooth (silver) gear without adapter is what was originally used in PRODUCTION. Of course, if you are using a tire size with an OD (or revs/mile) different than originally used for 1972, then your actual speedometer calibration will be off. Just how much depends how different your tire size is from that originally used.

For 1973-74 things were different. Instead of using a 22 tooth (silver) driven gear for 3.70 axle ratios, a 20 tooth (blue) driven gear was used in conjunction with the GM #6478879 adapter which had a ratio of 0.8824:1. I expect that this change between 1972 and 73-74 occurred because of the change in tire size between 1972 and 1973. Believe me, GM didn't use the adapter if they didn't need to; adapters are an expensive piece, even when used in PRODUCTION volume.

Also, I don't think that the GM #6477966 adapter was originally used for ANY 1972 Corvette. It was primarily used for SOME rather unusual pre-1971 applications with LOW numerical rear end ratios. I don't know how it wound up on your car. Bubba, I suppose. Certainly it would never have been used on a high numerical rear end ratio like 3.70:1.