ring and pinion gears

[Robert G.]

Hi All,
I have a '66 with a 3.70:1 positraction differential. I'm sure these gears were fun back in the car's early days with quick acceleration, but now not great for cruising.
The question is: are the physical size of these two unique to a Corvette or could a ring and pinion from another Chevy from the same period work?
Thanks for any and all help.
Rob

[Dick W.]

Re: ring and pinion gears

Hi All,
I have a '66 with a 3.70:1 positraction differential. I'm sure these gears were fun back in the car's early days with quick acceleration, but now not great for cruising.
The question is: are the physical size of these two unique to a Corvette or could a ring and pinion from another Chevy from the same period work?
Thanks for any and all help.
Rob

Unique to Corvette

[Michael J.]

Re: ring and pinion gears

I'm curious why you dislike a 3.70 rear end for cruising. I find it the most favorite one I have for my C2s, especially the fuelies.

[Robert G.]

Re: ring and pinion gears

I'm curious why you dislike a 3.70 rear end for cruising. I find it the most favorite one I have for my C2s, especially the fuelies.

Mike,
I live out your way, in the Durango area and the 3.70:1 makes the engine work too hard at highway speeds. I like to keep revs under 3K which makes the mph at around 60 or a little more. On occasion I have to drive to SLC, which is a long enough trip as it is.
So looking for a 3.36:1 or even 3.08:1, I want to keep the 3.70 for original equipment but for use now I'd like a lower ratio.

[Michael J.]

Re: ring and pinion gears

I see, thanks!

[Joe L.]

Re: ring and pinion gears

Mike,
I live out your way, in the Durango area and the 3.70:1 makes the engine work too hard at highway speeds. I like to keep revs under 3K which makes the mph at around 60 or a little more. On occasion I have to drive to SLC, which is a long enough trip as it is.
So looking for a 3.36:1 or even 3.08:1, I want to keep the 3.70 for original equipment but for use now I'd like a lower ratio.

Robert------


3.36:1 is the best overall rear gear ratio. However, with an L-79 with close ratio 4 speed, you might find a little "bog" getting going from a stop. It will be OK, though. For an L-36 or L-72, the 3.36 will work fine.

[Paul D.]

Re: ring and pinion gears

If, as you say, you want to retain the original 3.70 for later reinstallation, I would obtain a complete center section in the ratio you prefer. I think most of the venders offer them rebuilt in your choice of ratio. You would just have to pay the core charge or come up with one. Chip.

[Robert G.]

Re: ring and pinion gears

Robert------


3.36:1 is the best overall rear gear ratio. However, with an L-79 with close ratio 4 speed, you might find a little "bog" getting going from a stop. It will be OK, though. For an L-36 or L-72, the 3.36 will work fine.

Thanks Joe
Funny I do have the L79 w/close ratio.

[Michael J.]

Re: ring and pinion gears

Robert------


3.36:1 is the best overall rear gear ratio. However, with an L-79 with close ratio 4 speed, you might find a little "bog" getting going from a stop. It will be OK, though. For an L-36 or L-72, the 3.36 will work fine.

That's my problem with an L84, 3.08, and a close ratio, it is hard to even get it moving on hills from a stop, I can smell the clutch burning, so I upgraded to a 3.70 and love it. I have an L36 with a 3.08, and it will launch from 3rd gear if necessary on any hill, tons of low end torque. I have a 4.56 in my '63 L84, and it is wonderful, a solid lifter SHP engine sounds good at 3700 rpm, it seems to live at that RPM band.......

[Duke W.]

Re: ring and pinion gears

Mike,
I live out your way, in the Durango area and the 3.70:1 makes the engine work too hard at highway speeds. I like to keep revs under 3K which makes the mph at around 60 or a little more. On occasion I have to drive to SLC, which is a long enough trip as it is.
So looking for a 3.36:1 or even 3.08:1, I want to keep the 3.70 for original equipment but for use now I'd like a lower ratio.

The engine is turning relatively high revs compared to modern cars, but actually has very little load. You can verify by rigging up a vacuum gage in the cabin and will find that manifold vacuum at highway speed on a level road is quite high. The engine only needs to produce about 40 HP to move the car along, so though revs seem high, the load is light.

Your '66 should have the improved second design connecting rods that are much more durable than the first design that are subject to fatigue failure at the bolt seats, and there is no long run risk in cruising up to 4000+ revs on highways, which is about 85+ MPH with OE revs/mile tires.

Also, it sounds like you are at high altitude, so a taller gear will make the car more sluggish, especially starting off from a dead stop. Just drive it an enjoy the ride.

Duke

[Richard G.]

Re: ring and pinion gears

From Car and Driver's article from 1963.

"As the majority of new Corvettes are built with four-speed transmissions, it is hard to understand why the three-speed remains listed as standard equipment. We can see no reason for even continuing to offer it, and recommend that both the Powerglide and the three-speed manual gearbox be dropped. This would let Chevrolet standardize the wide-ratio four-speed transmission throughout and make the close-ratio version optional for the 340- and 360-bhp models."

My 63 340HP has the close ratio 4 speed. It is a bit of a chore to get it moving with the 3:55 gears. I agree the wide ratio should have been an option. If not for all ratios at least the ratios above 3:70's.

[Edward D.]

Re: ring and pinion gears

Your 3.70 is actually stock and optimal for your M21 close ratio transmission. This was the stock L79 configuration. That said, I run a 3.36 in my 66 L79 with an M20 wide ratio gearset in the transmission, this is another optimal configuration. The final 4th gear is 1:1 in both so on the highway the 3.36 turns lower RPMs. Using a 3.36 with a close ratio M21 will not work as well in the first couple of gears as the wide ratio gears but will work. There are several different gear sets available for the corvette rear end. One final warning is that changing the rear gears will change your speedometer reading, at a minimum you will need to change the plastic pencil gear, but if memory serves there are two different OD worm gears possible on the transmission tail shaft and the 3.70 uses a different one than the 3.36.

[James G.]

Re: ring and pinion gears

I loved the 3:55 gear with the wide ratio trans. I had that combo in a 66 AC convertible that was one of my favorite street cars. Off the line performance of a 4.11 CR, and great mid range cruising.

[Robert G.]

Re: ring and pinion gears

Thanks Jim, I have a close ratio trans. Talking with a company that makes the gears I was informed that going to a 3.36:1 wouldn't lower RPM's at highway speeds. So sticking w/3.70:1.

[Joe L.]

Re: ring and pinion gears

Thanks Jim, I have a close ratio trans. Talking with a company that makes the gears I was informed that going to a 3.36:1 wouldn't lower RPM's at highway speeds. So sticking w/3.70:1.

Robert------


Either whoever you talked to didn't know what they were talking about or you somehow misunderstood them. Changing from a 3.70:1 to a 3.36:1 (or any ratio numerically lower than 3.70:1) WILL lower highway RPM's and regardless of whether a wide or close ratio transmission is installed.