1970 ZR1 LOOKING for rare part

[Sal C.]

Re: 1970 ZR1 LOOKING for rare part

My 1970 ZR1 had a similar adapter. Had 4.56:1 ratio. I have a photo of it somewhere, I'll post if I can find it. Mean while here is a 6478879 which is for 1968 through 1975 4sp with 4.11:1

[Joe L.]

Re: 1970 ZR1 LOOKING for rare part

My 1970 ZR1 had a similar adapter. Had 4.56:1 ratio. I have a photo of it somewhere, I'll post if I can find it. Mean while here is a 6478879 which is for 1968 through 1975 4sp with 4.11:1

Sal-------

1970 with M-21 and 4.56:1 rear gear did use a speedometer adapter. It should be GM #6477965 with a ratio of 0.833:1. The adapter was necessary because the highest tooth count speedometer driven gear available was a 25 tooth gear. It would have required a non-existent 26 tooth gear for 4.56:1 if that was to be accommodated without an adapter.

I have no information that the GM #6478879 adapter was used for 1968-70 Corvettes with M-21 and 4.11:1 rear gear ratio. 1968-70 Corvettes with M-21 and 4.11:1 rear gear used a GM #3924097 speedometer DRIVE gear in conjunction with a GM #3860348, 25 tooth DRIVEN gear. This combination provides adequate speedometer calibration for Corvettes with F-70-15 tires.

For 1971-72, things changed. ALL 1971-72 Corvette 4 speed transmissions used a GM #3978758 DRIVE gear. This is a 1.77" OD gear and can only accommodate 18-22 (silver) tooth DRIVEN gears. The greatest tooth count gear of this series, the 22 tooth silver gear, will only provide speedometer accuracy without an adapter for 3.70:1 rear gear ratios with F-70-15 tires. So, 4.11:1 rear gear ratios require an adapter. For 1971-72 that adapter was GM #6477965 with a 0.833:1 ratio. 1971-72 uses a 21 tooth DRIVEN gear in combination with the adapter.

For 1973-74 things changed again. The transmission DRIVE gear remained the GM #3978578 which can only accommodate 18-22 (silver) tooth DRIVEN gears. For 1973-74 cars with 4.11:1 rear gears used the GM #3987922, 22 tooth DRIVEN gear in combination with the GM #6478879 adapter which is the adapter you have shown. I believe the difference between 1971-72 and 1973-74 is due to the different GR70-15 tire size used for those years.

[Sal C.]

Re: 1970 ZR1 LOOKING for rare part

1970 with 4.56:1, of which there were only about a half dozen, should have come with a 6477964 adapter.

[Joe L.]

Re: 1970 ZR1 LOOKING for rare part

1970 with 4.56:1, of which there were only about a half dozen, should have come with a 6477964 adapter.

Sal------


The GM information I have on that particular application is a little "non-definitive". Did your car have the 6477964 adapter installed and do you have a photo of it? The GM #6477964 adapter had a ratio of 1: 0.733. The GM #6477965 had a ratio of 1: 0.833.

[Ronald R.]

Re: 1970 ZR1 LOOKING for rare part

My car has a 411 with a M22 rock crusher, which one do I need? thanks RDR

[Ronald R.]

Re: 1970 ZR1 LOOKING for rare part

Per Joe I don't need one, my speedo was correct with out one before it broke. I am getting it repaired and will do a GPS check thanks RDR

[Ronald R.]

Re: 1970 ZR1 LOOKING for rare part

This chart shows I need a 6477965, please comment??? thanks Ronnie

[Joe L.]

Re: 1970 ZR1 LOOKING for rare part

This chart shows I need a 6477965, please comment??? thanks Ronnie

Ronald-------


The chart does NOT indicate that your application used a 6477965 adapter. If you had a wide ratio transmission (which was never actually available with a 4.11: 1 rear ratio), then you would need that adapter. An M-22 is a close ratio transmission. It does not require an adapter for 1970.

[Ronald R.]

Re: 1970 ZR1 LOOKING for rare part

Ohhhh, I see that, this might be a dumb question but the drive gear is on the tail shaft, what difference? Torque? thanks Ronnie

[Joe L.]

Re: 1970 ZR1 LOOKING for rare part

Ohhhh, I see that, this might be a dumb question but the drive gear is on the tail shaft, what difference? Torque? thanks Ronnie

Ronald------

It's on the main shaft. It has absolutely ZERO to do with torque or power (by the way the LT-1 has about the same torque as the base 350). It has to do with tooth count and OD of the drive gear.

[Ronald R.]

Re: 1970 ZR1 LOOKING for rare part

Ok thanks and I don't need a adapter so I'm happy !