FACTORY 308 12 BOLT GEARS

koolconvertible · Poncho Master!

RE: FACTORY 308 12 BOLT GEARS

cdnpont · Canadian Poncho Superstar!

3.36 would be a pretty reasonable gear in a BB/big car. I'm running 3.55's in my 496 TH400 2600 stall, can't get traction, and can't run the highway sustained.

But I have not seen any new 3.36 gearset available. Like I said, I think 3.23's would be the absolute perfect combo in an SB auto or M20 wide..but again...who makes it?

4SPEED427 · A Poncho Legend!

Yeah, I don't think there is a 12 bolt 3.23 gear out there. Honestly, I love my 3.07's. No, it's not a performer but it's tolerable at 100 km/h with my little 14" tires.

seventy2plus2 · Canadian Poncho Superstar!

cdnpont wrote:
How fast do you guys drive these things? I see 80 and some times up to 110 regularly and pretty quick in my 67, but I just kiss that number, then thats it, and back down she goes.
I figure 50 -70 is where most of the continuous running happens, with 55 probably being the comfortable sweet spot on the backroads around here.

When I do my long road trips in the USA I'm cruising at 80-85 mph along the Interstate Highways. I typically average 60 mph including fuel stops.

When I took my LS6 Parisienne to Iowa back in 2016, that big motor didn't like cruising at 3000 rpm, and started blowing oil out the front of the engine. A change in PCV valves, and a reduced speed to 60 mph limited the leak. This was all likely started by a groove in the harmonic balancer. It's been remedied with a speedy sleeve.

gparis7 · Canadian Poncho Superstar!

4SPEED427 wrote:
Yeah, I don't think there is a 12 bolt 3.23 gear out there. Honestly, I love my 3.07's. No, it's not a performer but it's tolerable at 100 km/h with my little 14" tires.

Did Gm install a 3.23 as original equipment? I can't recall.

I do remember there there being 2.73, 3.07, 3.31, 3.36, 3.42, 3.55 and 3.73.

4SPEED427 · A Poncho Legend!

I'll go out on a limb and say delete 3.36 and 3.42 from your list, add 2.29 and 2.56 as factory 12 bolt gear ratios in passenger car diffs. That and of course there's the 4 series gears still.

cdnpont · Canadian Poncho Superstar!

seventy2plus2 wrote:
cdnpont wrote:
How fast do you guys drive these things? I see 80 and some times up to 110 regularly and pretty quick in my 67, but I just kiss that number, then thats it, and back down she goes.
I figure 50 -70 is where most of the continuous running happens, with 55 probably being the comfortable sweet spot on the backroads around here.
When I do my long road trips in the USA I'm cruising at 80-85 mph along the Interstate Highways. I typically average 60 mph including fuel stops.
When I took my LS6 Parisienne to Iowa back in 2016, that big motor didn't like cruising at 3000 rpm, and started blowing oil out the front of the engine. A change in PCV valves, and a reduced speed to 60 mph limited the leak. This was all likely started by a groove in the harmonic balancer. It's been remedied with a speedy sleeve.

Now these are the stories I like to hear. Some high speed and the results in these big well powered cars.

I'd love a OD in my 67.

HK1837 · Addicted!

gparis7 wrote:
4SPEED427 wrote:
Yeah, I don't think there is a 12 bolt 3.23 gear out there. Honestly, I love my 3.07's. No, it's not a performer but it's tolerable at 100 km/h with my little 14" tires.
Did Gm install a 3.23 as original equipment? I can't recall.
I do remember there there being 2.73, 3.07, 3.31, 3.36, 3.42, 3.55 and 3.73.

GM Engineering Technical Specs for '69 Camaro 8.875" ring gear list:

2.56

2.73

3.07

3.31

3.55

3.73

4.10

4.56

4.88

There is a 2.29 listed for full size but it is an 8.625" ring gear however I think that is still a 12-bolt. Someone else here will know more.

67Poncho · Uber Guru

Looked in my '58-'68 #682 catalogue and only '66 diffs are listed in that one.. I know I have 3:31's in my '67 2+2 Rag and they were pilfered from a '67 4-dr Strato. I have a M20 and 235/75 15 tires and she buzzes at @ 3000 at 70mph. I have told myself that I need to change them to a 3:07 for years but I still haven't done that. (25 years and counting)

-- Edited by 67Poncho on Wednesday 5th of August 2020 08:26:44 AM

unruhjonny · Guru

Yes, I realize I'm a little late to the party...

I'm glad to see the gear ratio was corrected early on.

I always found it curious that specific gear sets were for specific rear ends;
It's not like GM couldn't have made the same ratio gears work in multiple differential styles.
As examples:
The corporate car 10 bolt used 3.08/3.42
While the corporate car 12 bolt got 3.07/3.55

My car has had the whole time I have owned it, a correct as built 12 bolt with 3.55's and I personally never felt it was revving too high, then again, I never cruised at 80mph (if my math is correct 80mph is nearly 130kph);
I did though on one of my drives between Winnipeg and Calgary cruise at an average of 55mph - and with a 1977 Pontiac carb in stock tune, actually exceeded 20mpg... which in and of itself was astonishing since a year correct carb for my car with average driving pegs it more around 9mpg's.

lol

I have a spare 12 bolt I recently acquired (anyone want a used 3.31:1 ring and pinion?), that I will be setting up with a 3.90:1 gear set;
Go ahead and laugh, but here's my reasoning;
I had one, singular, brief ride in an authentic 1968 Firebird 400;
Now, this was a 100% legit 'Ram Air' car...
On paper that car's drivetrain was every part my cars equal (it was a D-port "Ram Air I", not a round-port "Ram Air II");
Compared to my car, this thing was beastly;
The only significant deviation from my car was the factory set of 3.90:1 gears.
I could set up a set of 1970-correct 3.73:1 (I have those too, and occasionally I list them on kijiji), but that would still give that '68 Firebird a theoretical "leg up" on my car. ;)

And incase you care to know, the same year GTO's with the 'Ram Air' engine option, received 4.30:1 gear sets.