Speedometer adapter for 72 LT-1 confusion

[Gary S.]

My 5th Edition TIM&JG says that "those cars with either 4:11 or 4:56 axle ratios have a reducing gearbox attached to the speedometer cable outlet of the transmission." Then I found this Wilcox Corvette's link http://repairs.willcoxcorvette.com/w...o-chart-mt.png with regard to 1970 Corvettes. Then I read Joe Lucia's comment on 69 Corvettes https://www.forums.ncrs.org/showthre...ometer+adapter. (Still reading items from my search, btw).

Am I comparing apples to oranges here? My speedo cable was bouncing and the transmission end is buggered up so I am replacing it and in the process I removed a 6477965 reducing gearbox from the car. Wilcox's chart seems to suggest that the 70s did NOT have the reducer on 4.11s but the 70-72 TIM&JG states otherwise.

I am the fifth owner of this 43K mile 72 LT-1 with Close Ratio 4.11 differential and discussions with previous owners 2-4 say that the only thing changed on this car was normal items. were the 69-70s different enough from the 71-72 model or am I missing something very obvious?

Thank you in advance,
Gary

[Joe L.]

Re: Speedometer adapter for 72 LT-1 confusion

My 5th Edition TIM&JG says that "those cars with either 4:11 or 4:56 axle ratios have a reducing gearbox attached to the speedometer cable outlet of the transmission." Then I found this Wilcox Corvette's link http://repairs.willcoxcorvette.com/w...o-chart-mt.png with regard to 1970 Corvettes. Then I read Joe Lucia's comment on 69 Corvettes https://www.forums.ncrs.org/showthre...ometer+adapter. (Still reading items from my search, btw).

Am I comparing apples to oranges here? My speedo cable was bouncing and the transmission end is buggered up so I am replacing it and in the process I removed a 6477965 reducing gearbox from the car. Wilcox's chart seems to suggest that the 70s did NOT have the reducer on 4.11s but the 70-72 TIM&JG states otherwise.

I am the fifth owner of this 43K mile 72 LT-1 with Close Ratio 4.11 differential and discussions with previous owners 2-4 say that thee only thing changed on this car was normal items. were the 69-70s different enough from the 71-72 model or am I missing something very obvious?

Thank you in advance,
Gary

Gary-------

For 1970 and prior, there were 2 different speedometer DRIVE gears available to cover rear gear ratios of 3.08:1 to 4.11:1 . Each of these could be paired with several different speedometer DRIVEN gears depending on which rear gear ratio was installed. These gears ranged in tooth count from 18 to 25. The 18 to 21 tooth DRIVEN gears were generally used for 3.08:1 to 3.56:1 rear gear ratios. The 22 tooth (green) to 25 tooth DRIVEN gears were generally used for 3.70:1 to 4.11:1. Ratios below 3.08:1 and higher than 4.11:1 required a speedometer adapter in addition to specified DRIVE and DRIVEN gears.

For 1971 and later, only one speedometer DRIVE gear was available. This gear was compatible with 18-21 tooth DRIVEN gears and accommodating 3.08:1 to 3.56:1 rear gear ratios. In addition, however, a new 22 tooth (silver) gear was released which was compatible with the only available DRIVE gear and could accommodate 3.70:1 rear gear ratios. No rear gear ratio numerically higher than 3.70:1 could be accommodated with any combination of available DRIVE/DRIVEN gears. So, any gear ratio numerically higher than 3.70:1 had to use a speedometer adapter to achieve accurate speedometer calibration.

[Gary S.]

Re: Speedometer adapter for 72 LT-1 confusion

Thank you, Joe. I think I got that information. Btw, it seems I asked a similar question in March 2002. CRS disease? Lol.

Now the water becomes even murkier for me. I just pulled my DRIVEN gear out and it is brown/18 tooth. This is with a 4.11 and Close Ratio transmission. Your information says that this Driven gear is not original to this un-rebuilt and original transmission. 20 +/- years ago I switched from the original 4.11 to 3.36 differential and I assume that I changed the DRIVEN gear at that time but I have no recollection of it. The differential has been returned to its original 4.11 gear set with a complete rebuild. (Nice job, Brian Bair.) The speedometer reads a constant 5 mph slow at GPS measured speeds of 30, 35, 40, 45 mph. I know that Duke Williams said that there are two types of errors in speedometers one that is constant (static error maybe?) and the other was speed proportional. Are these two connected? Or is my thinking flawed?

As always, thank you for your valuable assistance.

Gary

edit - I started thinking about this and if the DRIVEN gear was off then the speed would be off by a speed dependent percentage, not a fixed number like my 5 mph. Again, is my thinking flawed?

[Joe L.]

Re: Speedometer adapter for 72 LT-1 confusion

Thank you, Joe. I think I got that information. Btw, it seems I asked a similar question in March 2002. CRS disease? Lol.

Now the water becomes even murkier for me. I just pulled my DRIVEN gear out and it is brown/18 tooth. This is with a 4.11 and Close Ratio transmission. Your information says that this Driven gear is not original to this un-rebuilt and original transmission. 20 +/- years ago I switched from the original 4.11 to 3.36 differential and I assume that I changed the DRIVEN gear at that time but I have no recollection of it. The differential has been returned to its original 4.11 gear set with a complete rebuild. (Nice job, Brian Bair.) The speedometer reads a constant 5 mph slow at GPS measured speeds of 30, 35, 40, 45 mph. I know that Duke Williams said that there are two types of errors in speedometers one that is constant (static error maybe?) and the other was speed proportional. Are these two connected? Or is my thinking flawed?

As always, thank you for your valuable assistance.

Gary

edit - I started thinking about this and if the DRIVEN gear was off then the speed would be off by a speed dependent percentage, not a fixed number like my 5 mph. Again, is my thinking flawed?

Gary------

I cannot find that an 18 tooth DRIVEN gear was used for 1972 with 4 speed and 4.11:1 rear gear. It was not used for that same application with 3.36:1 rear gear, either.

My information shows that 1972 with 4 speed and 4.11:1 rear gear used a 21 tooth red DRIVEN gear in combination with a GM #6477965 adapter. However, it is possible that an 18 tooth DRIVEN gear could have been used with a different adapter. But, I have nothing to confirm that. There is no way that a 1972 with 4.11:1 rear gear could have been built without an adapter and certainly not with an 18 tooth DRIVEN gear without an adapter.

A 1972 with 3.36:1 rear gear should use a 21 tooth red DRIVEN gear without an adapter.

All of the above assumes that the car has F-70-15 size tires OR other tires with equivalent revs/mile.

[Gary S.]

Re: Speedometer adapter for 72 LT-1 confusion

As usua, Joe you provide exceptionally detailed information! Thank you. I'll get a red gear in there next week.

Gary

[Joe L.]

Re: Speedometer adapter for 72 LT-1 confusion

As usua, Joe you provide exceptionally detailed information! Thank you. I'll get a red gear in there next week.

Gary

Gary------

You'll still need a correct speedometer adapter in conjunction with the 21 tooth gear. The original adapter was of ratio 1: 0.833 and of GM #6477965 but it's long-since GM discontinued.

[Gary S.]

Re: Speedometer adapter for 72 LT-1 confusion

I have my 6477965 adapter.

Thanks for the follow-up

[Joe L.]

Re: Speedometer adapter for 72 LT-1 confusion

I have my 6477965 adapter.

Thanks for the follow-up

Gary------


Please post photos of the adapter.

[Gary S.]

Re: Speedometer adapter for 72 LT-1 confusion

Here you go. I am still reading about these and have seen photos that don't look anything like mine but seem to have been dismissed as not typical. Oth, this was on my car 21 years ago but the qualifier is that I am not the original owner and the cable has probably been changed because the end that does in the adapter is buggered up.

Gary

[Joe L.]

Re: Speedometer adapter for 72 LT-1 confusion

Here you go. I am still reading about these and have seen photos that don't look anything like mine but seem to have been dismissed as not typical. Oth, this was on my car 21 years ago but the qualifier is that I am not the original owner and the cable has probably been changed because the end that does in the adapter is buggered up.

Gary

Gary-------


The speedometer adapters used in PRODUCTION and once available in GM SERVICE were manufactured by Stewart-Warner, AC Spark Plug Division, and, possibly, others. Often times, but not always, they have the ratio stamped on the body.

[Joe L.]

Re: Speedometer adapter for 72 LT-1 confusion

Gary------


Attached are photos of an NOS GM #1565208. This is the adapter that replaced the GM #6477965 in 1976. It's also been discontinued now for several years.


DSCN3975.jpgDSCN3976.jpgDSCN3977.jpgDSCN3978.jpg

[Gary S.]

Re: Speedometer adapter for 72 LT-1 confusion

That is interesting in that it looks to be rebuildable whereas its predecessor does not.