Speedometer cable adapter

[Gary S.]

Hi to all of you. Today I was discussing differential gear ratios with a friend and we got into whether or not the 90 degree adapter that is on the transmission end of my speedometer cable is on all 72 and up cars? My car is a 72 with 4.11 gears and close ratio transmission and has the adapter. My friend thinks that ALL Corvettes from around 71 had this adapter, up through mid to late 70's. He claims that this was designed to reduce stress where the cable goes into the transmission. I thought that I had read on this board that the 90 degree adapter was only used on cars with 4.11 or higher differential ratios. I checked the archives and my AIM and I don't see any reference to these adapters.

Does anyone have the definitive answer to this issue?

[Joe L.]

Re: Speedometer cable adapter

Gary-----

Beginning in 1971, Muncie transmissions used only one speedometer drive gear (the gear on the transmission mainshaft). This gear was of the same diameter as the "larger" version of the 2 gear diameters used on 1970 and older Corvettes. Consequently, it would mate with only the "smaller" series of speedometer driven gears. This change, which greatly reduced the number of transmission part numbers needed for PRODUCTION, also left higher numerical gear ratios "out in the cold" with respect to a workable driven gear for an accurate speedometer. The solution was speedometer adapters for certain gear ratios, usually 4.11 and above and, sometimes, 2.73 and lower. Speedometer adapters are an expensive solution but, apparently, GM felt that the cost was worth the reduction in PRODUCTION transmission part numbers. Plus, they were only needed for the rather low-production gear ratios, anyway.

So, speedometer adapters were used on only SOME 71+ models (they were also used on a few pre-71 applications, too). Several adapters were used with different gear ratios, usually in the 0.8 to 1.1 range, and some were 90 degree and some were not. There was no 1:1 adapter with 90 degree fittings, so cars not requiring external gearing (an adapter) for an accurate speedometer could not have used an adapter. This would have included most 3.08 to 3.70 rear gear ratios. For 1971+, GM developed a "small diameter" 22 tooth driven gear, previously only available with the "large" series of driven gears, so that most 3.70:1 applications could be accomodated by an available gearset of the "large" drive gear/ "small" driven gear series and without the need for an adapter. This 22 tooth driven gear is GM #3987922 and is color-coded "silver".

[Gary S.]

Re: Speedometer cable adapter

Thank you, Joe. I was more or less certain that I had remembered a post to this effect quite a while ago. Unfortunately, I didn't archive it myself and relied on memory.

The other thing that I remember is that if one were to pull out these numerically higher gear ratios then the adapter must be removed, the proper gear must go in the transmission, and, as a result of losing the adapter, the cable may not reach the connection to the transmission.

Gary

[Harmon C.]

Re: Speedometer cable adapter

If you have a GM Corvette parts catalog the adapters and their usage are listed on page 4-5C 4.000 group. From 73 to 77 3.36 ratio and higher used an adapter in most years with the 4 speed transmission.

[Gary S.]

Re: Speedometer cable adapter

Great information! Thanks. I don't have the GM Corvette parts book so I have to rely on guys like you, Joe, and others. Much appreciated. I can assume that if the 3.36 for 73 through 77 needed the adapter, then the same thing would apply to 72.
Gary

[Robert C.]

Re: Speedometer cable adapter

Gary, There was a metal adapter on my 71 w/ 4-11 rear end. It mates to a plastic gear of x number of teeth to make the speedo work right. My plastic gear was orange.

[Joe L.]

Re: Speedometer cable adapter

Gary-----

For 1971 and 1972 Corvettes with 4 speed, only cars equipped with 4.11:1 and 4.56:1 axle ratios used adapters. All other ratios did not use an adapter. For 73+, adapters were used for most ratios. This probably had something to do with the change in tire size for 1973. The adapter used for most 73+ 4 speed applications had a .8824:1 gear ratio.

[Pat Bush #35083]

Re: Speedometer cable adapter

Gary -

Been through this with my 73 already. Mine has a 3.36 and has the 90 degree adapter. BTW mine took a Brown gear and now the speedo actually reads correctly. There seemed to be some discrepancy on the board with exactly when and where the adapter came into play. If you search the archives round about March of last year, there are several posts that deal with my car from Joe Lucia and others on gear selection as well as more detail on the adapter itself.

Pat

[Joe L.]

Re: Speedometer cable adapter

Bob-----

Your last sentence surprises me. For a 1971 with 4.11:1, you should have a GM #6477965 adapter (may be stamped "Stewart-Warner 8.000")and a RED driven gear of 21 tooth count and of GM #3987921. The ORANGE driven gear is a gear of 25 tooth count which is NOT compatible with the GM #3978758 speedometer drive gear used on all 1971+ 4 speeds. The 3978758 gear is the only available drive gear which will fit on the 71+ mainshaft, so it's not possible for someone to have previously chnaged that.

If the driven gear is, indeed, an ORANGE gear of 25 tooth count, then either you have the wrong gear driven gear installed in the transmission OR you have an earlier transmission. Your transmission should have the casting number 3978764 on the extension housing and, if so, then you should have the 21 tooth RED driven gear installed in the transmission.

It's possible that your RED driven gear has faded to ORANGE after years in the transmission. However, the tooth count should confirm its configuration and that should be 21.

[Gary S.]

Re: Speedometer cable adapter

So, if 73 and later cars with at least 3.36 gears had this 90 degree, can one assume that the adapter has a fixed ratio or is it merely a 90 degree adapter to reduce stress? Previous comments on this thread would indicate that the adapter is more than just a 90 degree change on the cable.

Gary

[Joe L.]

Re: Speedometer cable adapter

Gary-----

The most widely used 73+ adapter had a gear ratio of 0.8824:1. As far as I know, there never was an "adapter" of 1:1 ratio and 90 degree configuration.

[Gary S.]

thanks to all *NM*