Differential Question

[Junky]

The differential in my 1962 327/300 BelAir wagon has a BB code stamped into the differential case, and when I look it up in the reference page, it appears that it is a 3:36 based on the BB stamping. The tag on it has 2 holes in it, indicating that it is a 3:55. The date code on the differential is consistent with the build date of the car. Being that I don't know all the history of the car, I was wondering if anyone knows what the original gear ration that the car would have come with. I don't have a tach installed in the car, however, at 65 MPH, the engine seems to be reving quite high, and for that reason I would like to change it to a 3:08 gear. What are your thoughts on my proposed change?

 

[Iowa 409 Guy]

Miles driven annually?

 

[1964SuperStocker]

Use of the car is the most important part. Not to mention which transmission are you running. I'm working on putting a 700r4 in my wagon because I plan to drive it daily this year. If you want better fuel economy the best way is a 700r4 because of the 4 speeds and locking overdrive. Putting a 3.08 vs a 3.36 or 3.55 isn't going to make much difference. Maybe you would get 1mpg better with a 3.08 in town. Maybe 2mpg on the highway. Changing the rear gear would be cheaper than a new transmission but that is about it. Even switching to a turbo350 transmission over a powerglide would be an improvement.

 

[64ss409]

Wouldn't your tag be the second one? 3.36?

 

[Don Jacks]

Set up your tune up tach and see what the actual rpm actually is at 65.Most of us have become so used to modern cars and trucks with their overdrive transmissions that we "think"that the older cars are really buzzing with their 3.36 gears and their 24-25 inch tall tires when that's the way these cars were designed to run.Switching from a Glide to a 350 will not make any difference in road rpm because both are 1-1 in high gear.Switching from 3.36 to 3.08 will only drop tour cruising rpm by about 245 rpm,If your cars tire diameter is 25 inches,your engine will be turning just under 3,000 rpm at 65 mph assuming no converter slip.A 700r4 trans.,in top gear with the clutch engaged will drop your rpm to just under 2,100 with the gear/tire set up you currently have. You can use this formula for gearing: mph x gear x 336 divided by tire diameter to get your rpm at a given speed.

 

[409gang]

Set up your tune up tach and see what the actual rpm actually is at 65.Most of us have become so used to modern cars and trucks with their overdrive transmissions that we "think"that the older cars are really buzzing with their 3.36 gears and their 24-25 inch tall tires when that's the way these cars were designed to run.Switching from a Glide to a 350 will not make any difference in road rpm because both are 1-1 in high gear.Switching from 3.36 to 3.08 will only drop tour cruising rpm by about 245 rpm,If your cars tire diameter is 25 inches,your engine will be turning just under 3,000 rpm at 65 mph assuming no converter slip.A 700r4 trans.,in top gear with the clutch engaged will drop your rpm to just under 2,100 with the gear/tire set up you currently have. You can use this formula for gearing: mph x gear x 336 divided by tire diameter to get your rpm at a given speed.

Your right people are so use to having low engine RPM today as all vehicles have OD now. Only us old farts remember engines buzzing at 2700 RPM or more @ 70 mph depending on the gear you had. The only way to have the performance of a vehicle with a low axle ratio and low RPM @ highway speeds is to put an OD transmission in as all transmissions are 1:1 in final gear unless it has OD.

 

[Junky]

The reason for wanting to go to the 3:08 gears is that I recently picked up a P case that is correctly date coded for my car, and it was originally a 3:08 according to the BX code stamped into the case. I bought it last summer on Facebook, sight unseen, but the seller claimed that it was rebuilt about 10 years ago, but never installed in his Corvette, which he sold. I had a friend pick it up for me and store it until we met up in Michigan last weekend. I dropped it off at a friend who was going to check the pattern, and he called to say that the back lash was .020, and that it looked like it might have been a swimmer. I picked up the pieces today, and no doubt that it sat in water at one time. I now have an expensive P case with nothing to put into it. It was suggested that I purchase a new posi unit and a new gear set. I want to do this only once, and quality and quietness is paramount. Suggestions are welcome. thanks Junk
PS.. do you think that this old posi unit (without case) has any value??

 

[Don Jacks]

Yes it has value.It's rebuildable and it can easily be used in a non P case with minor mods to the case.

 

[Cayneman]

There's really not that much difference between a 3.36 and a 3.08. I wouldn't waste my time and money.

 

[1964SuperStocker]

Set up your tune up tach and see what the actual rpm actually is at 65.Most of us have become so used to modern cars and trucks with their overdrive transmissions that we "think"that the older cars are really buzzing with their 3.36 gears and their 24-25 inch tall tires when that's the way these cars were designed to run.Switching from a Glide to a 350 will not make any difference in road rpm because both are 1-1 in high gear.Switching from 3.36 to 3.08 will only drop tour cruising rpm by about 245 rpm,If your cars tire diameter is 25 inches,your engine will be turning just under 3,000 rpm at 65 mph assuming no converter slip.A 700r4 trans.,in top gear with the clutch engaged will drop your rpm to just under 2,100 with the gear/tire set up you currently have. You can use this formula for gearing: mph x gear x 336 divided by tire diameter to get your rpm at a given speed.

The turbo 350 is an improvement in every aspect except the final drive. First gear in a glide is 1.76/1.82 and Turbo 350 is 2.52 so performance up to before you get to final drive with a 350 would in fact be better. That means all stop and go traffic including your normal running around town would make a difference. The addition of any gears more than a power glide has is a significant improvement really. That would include should you need to down shift for any reason then your engines rpm wouldn't need to redline because you have more gears to choose from.
Use the formula Don pointed out. Its helpful to have a solid understanding of what RPM you are at to determine where you may want to be. I am running a 3.70 posi in my wagon because with my powerglide I have more weight to get moving so the lower gear is essential. Plus I've always known I would put a 700r4 in the wagon to get this heavy boat to pull the whole family down the road with all of the kids junk and the 9 passenger seating set up in it with A/C. The car will be around 4400 lbs once everything is loaded without people in it. In its trimed down form its at 4040lbs.
NUMBERS MATCHING? None of what we say matters then. LOL!

 

[Junky]

.........
NUMBERS MATCHING? None of what we say matters then. LOL!

I do listen, and I do respect the opinion of others, and I also explain why I am staying on the road that I have chosen. My car is probably 1 out of a possible 15 or 20 built, and it is the only known survivor. For that reason, I prefer to keep it as original as possible. I have spent a lot of energy restoring, and I intend to stay the course that I have chosen.

 

[Junky]

Wouldn't your tag be the second one? 3.36?

Yes, it would be, however, I have a very strong suspicion that the rear was changed prior to my ownership. If I had to guess, and it would only be a guess, based on the evidence of having a tag with 2 holes in it, I believe that the original ratio was a 3.55.

 

[1964SuperStocker]

I do listen, and I do respect the opinion of others, and I also explain why I am staying on the road that I have chosen. My car is probably 1 out of a possible 15 or 20 built, and it is the only known survivor. For that reason, I prefer to keep it as original as possible. I have spent a lot of energy restoring, and I intend to stay the course that I have chosen. View attachment 79176

Look out! Professional car coming through! I'm guessing you are a member of the http://www.professionalcarsociety.org/ http://www.theprofessionalcarsociety.org/
Great group of knowledgable folks. I ran the professional car class at the Des Moines Concours a couple years back and would have loved to have shown your 62'. I know where there are dozens of professional cars due to my year long search to fill my class. They are just as a passionate bunch as any group. I bought all of the extra gear needed to make my wagon a "fire chief" clone professional car with all of the lights, sirens and radios. Even had the shield design picked out. Then I came to my senses and decided it would be more fun to just set it up to run down the track now and then.

 

[1964SuperStocker]

I do listen, and I do respect the opinion of others, and I also explain why I am staying on the road that I have chosen. My car is probably 1 out of a possible 15 or 20 built, and it is the only known survivor. For that reason, I prefer to keep it as original as possible. I have spent a lot of energy restoring, and I intend to stay the course that I have chosen. View attachment 79176

Forgot to ask. Who did the coach work? I'm guessing you have some history on her service years too?

 

[Junky]

Coach was built by Cotner Bevington in Blytheville Arkansas. Was originally in Service in Nebraska.