Need help ID'ing Speedo adapter

[Gary R.]

I have been parting out our 75 L-82 and we have been installing some parts on the new 75 we have.

The question I have is the new L48 4 speed 336 car has a speedo adapter on it while the L82 4 speed car with 355's did not.

Can someone ID this adapter and tell me why it was used in this application?

Thanks

[Joe L.]

Re: Need help ID'ing Speedo adapter

I have been parting out our 75 L-82 and we have been installing some parts on the new 75 we have.

The question I have is the new L48 4 speed 336 car has a speedo adapter on it while the L82 4 speed car with 355's did not.

Can someone ID this adapter and tell me why it was used in this application?

Thanks

Gary------


This adapter is a 0.8824:1 ratio.

I get conflicting information on whether an adapter was used for this application. One source indicates no adapter was used. Another source indicates that adapter GM #6478879 was used with all 1975 manual transmission applications except 3.08:1.

Curiously, the 6478879 was discontinued from SERVICE in October, 1976 and replaced by the GM #1565211, the very adapter you have. It's also very possible that the 1565211 was used in PRODUCTION in place of the 6478879 at some point during the 1975 model year. Or, it could be that the original was replaced for some reason.

As to why it was used, I can only speculate. For whatever reason, the use of any available DRIVEN gear with the only DRIVE gear used for all 1975 4 speed applications must not have produced an acceptable speedometer calibration error. However, a certain DRIVEN gear in combination with the 0.8824 adapter apparently did result in an acceptable calibration. The DRIVEN gear that was supposed to have been used with 3.36:1 rear ratio and this adapter is GM #3987918, an 18 tooth brown gear.

However, according to the information I have, the same adapter should have been used with a 3.55:1 rear gear ratio and M-21 transmission. In that case, though, it would have been used in conjunction with a 3987919, 19 tooth natural color DRIVEN gear.

There had to be a good reason for GM to use an adapter. Ordinarily, they could be expected to do everything possible to avoid it because adapters are quite expensive.

[Gary R.]

Re: Need help ID'ing Speedo adapter

Hi Joe
Thanks for the info. I had this at Carlisle in case you were there and stopped by the spot again.
From what I can tell this is original to the M-20/336 car, it only has 51k miles on it and I have history on it.
The 75 L82 car was also stone stock when I bought it. It came with the M-20 and 355's,no adapter.
I will be selling the 336 M-20, I will probably go through it and rebuild/clean it so I'll check the speedo gears to see what is there.

Thanks Joe.

[Michael W.]

Re: Need help ID'ing Speedo adapter

Gary,

My '73 with M20 and 3:36 has that same adapter, with a brown 19 tooth driven gear. Speedo accuracy as verified by a GPS is within 1% or so, speedo reads high at all times.

I'd say yours is original.

[Gary R.]

Re: Need help ID'ing Speedo adapter

Mike
I agree but the question is why on one and not the other,same trans unless the drive gears are different.

[Joe L.]

Re: Need help ID'ing Speedo adapter

Hi Joe
Thanks for the info. I had this at Carlisle in case you were there and stopped by the spot again.
From what I can tell this is original to the M-20/336 car, it only has 51k miles on it and I have history on it.
The 75 L82 car was also stone stock when I bought it. It came with the M-20 and 355's,no adapter.
I will be selling the 336 M-20, I will probably go through it and rebuild/clean it so I'll check the speedo gears to see what is there.

Thanks Joe.

Gary-----


I was not at Carlisle this year.

[Joe L.]

Re: Need help ID'ing Speedo adapter

Gary,

My '73 with M20 and 3:36 has that same adapter, with a brown 19 tooth driven gear. Speedo accuracy as verified by a GPS is within 1% or so, speedo reads high at all times.

I'd say yours is original.

Mike-----


Are you sure the gear is BROWN and/or that it's 19 tooth? The BROWN gear is 18 tooth. The "NATURAL" gear (may appear light brown after long contact with gear oil) is 19 tooth. You M-20 with 3.36 should have the adapter and an 18 tooth DRIVEN gear.

[Joe L.]

Re: Need help ID'ing Speedo adapter

Mike
I agree but the question is why on one and not the other,same trans unless the drive gears are different.

Gary----


One source indicates that neither the M-20 or M-21 with either 3.36:1 or 3.55:1 used an adapter. The other source indicates that BOTH used an adapter. So, why one would have an adapter and the other not, I do not know. It might be that the 3.36:1 required an adapter to get within an acceptable limit of speedometer error whereas the 3.55:1 did not require the adapter to get within an acceptable limit of speedometer error.

The way I figure it, the use of an 18 tooth DRIVEN gear with an 0.8824:1 adapter produces the equivalent of a "20.4 tooth" DRIVEN gear. It may be that with the GR70-15 tire size and 3.36:1 rear gear, a "20.4 tooth" DRIVEN gear was required. Obviously, that could not be achieved, so the 18 tooth DRIVEN gear with 0.8824:1 adapter was used.

The 3.55:1 rear gear with GR70-15 tires might have achieved acceptable speedometer calibration with a DRIVEN gear of "whole number" tooth count. If so, I would expect that a RED 21 tooth gear or, less likely, a SILVER 22 tooth gear.

For 1971-74 Muncie transmissions, there was only one DRIVE gear available. This was GM #3978758. It was a STEEL gear or 8 tooth count. It was compatible with the "small" series of DRIVEN gears of 18-22(silver) tooth count. For 71-74 Muncies, any use of a rear gear ratio requiring a 23 tooth or greater DRIVEN gear had to be accommodated by use of an adapter of appropriate ratio with a 22 tooth or less DRIVEN gear.

For 1975-81 Corvettes with Borg Warner transmission, at least 3 different DRIVE gears were available. There was one gear that was SUPPOSED to have been used for all 1975 4 speed applications. This was an 8 tooth BLUE gear of GM #6260705. It was compatible only with the 18-22(silver) tooth DRIVEN gears.

Other DRIVE gears that may have been used during the 75-81 period for B-W T-10 as well as the 78-79 Muncie M-20 are as follows:

GM #6261784----8 tooth ORANGE
GM #6261797----7 tooth BROWN
GM #6261795----
GM #6261796----9 tooth RED
GM #361002-----7 tooth GREEN
GM #6261794----8 tooth NATURAL

All of the above are also compatible ONLY with the 18-22(silver) DRIVEN gears.

[Michael W.]

Re: Need help ID'ing Speedo adapter

Mike-----


Are you sure the gear is BROWN and/or that it's 19 tooth? The BROWN gear is 18 tooth. The "NATURAL" gear (may appear light brown after long contact with gear oil) is 19 tooth. You M-20 with 3.36 should have the adapter and an 18 tooth DRIVEN gear.

Good catch Joe, it is the brown gear with 18 teeth.