C3 Speedometer (ratio) adapters on manual trans

[Wayne M.]

I think they started in 1968; was this concurrent with the switch to plastic mainshaft drive gear ? ie. to minimize the number of drive/driven gears for certain odd-ball final drives.

Were they Stewart-Warner manufacture (thumbnail attached), or GM in-house (catalog Gr 4.329) ?

[William C.]

Re: C3 Speedometer (ratio) adapters on manual trans

They were a catalog item, but all the ones I have seen were Stewart-Warner sourced. Usually they were used when an odd rear end gear was used and the available combinations with internal driven-drive gears could not be matched to the required speed shaft speed to achieve a correct speedo reading. I think my W-30 Olds with 3.91 gearing used one, and some of the less common Corvette apps call for them also. My '65 parts book indicates the adapters were in common use for Commercial vehicles as early as the 1941 timeframes, Generally they were available as 12% step up or 6% stepdown, or the reverse, as the parts book does not indicate in and out.

[Dick W.]

Re: C3 Speedometer (ratio) adapters on manual trans

S S White picked up the manufacture of these adapters when SW discontinued them.

[Joe L.]

Re: C3 Speedometer (ratio) adapters on manual trans

I think they started in 1968; was this concurrent with the switch to plastic mainshaft drive gear ? ie. to minimize the number of drive/driven gears for certain odd-ball final drives.

Were they Stewart-Warner manufacture (thumbnail attached), or GM in-house (catalog Gr 4.329) ?

Wayne-----


The nylon speedometer drive gears were used for 1968-70 manual transmission applications. However, there were at least 2 different drive gears used and these were equivalent to the earlier steel drive gears of GM #3708144 and 3708145. So, the change to nylon drive gears, by itself, did not necessitate any wider use of adapters.

However, for 1971-74 there was only ONE drive gear available. This was a steel drive gear of GM #3978758. It was compatible with only the 18-22 (silver) tooth driven gears. So, if any particular rear gear ratio required any other tooth count driven gear, an adapter had to be used. Of course, during the 1971-74 period, there were few gear ratios available that required anything other than the 18-22 (silver) tooth driven gears.

By the way, the nylon drive gears used for 1968-70 did return for one more "encore" appearance during the C3 years. This was for the "Muncie" transmission used for 1978-E79 Corvettes with L-48 and 4 speed.