Can someone please explain the final gear ratios?

2.72 
  
3.08
  
3.23
  
3.36
  
3.55
  
3.70 
  
3.90
  
4.11
  
4.33

Which one is the most ideal for fast take off, and high top speed? I've been told its the 3.90, is this true? What is the power loss with each of them?

I have a 1977 L48, it was manufactured in October of 1976, what gear shoudl I have?

Thanks guys,
-Louis

 

new77guy38756.6493518519

Louis,

I believe you should have one of the following:

1. OA = 3.08

2. LR,OD = 3.36

3. OB = 3.55

4. OC = 3.70

Not sure what exact you have on your prefix number. 

Where can I find this number without dropping the differential?

[QUOTE=new77guy]Where can I find this number without dropping the differential?[/QUOTE]

The serial numbers are located on the bottom surface of the carrier at the cover mounting flange. 

Hope this helps you out.

'77, L-48, with an automatic will only have a 3:08 rear gear ratio. There were no rear-end options on the auto. trans. cars(L-48).
The letter code "OA" will be stamped in the very bottom of the diff., just ahead of where the spring bolts on, and the two halves of the diff. are bolted together. It is hard to see on most cars, but a good cleaning will reveal it. There will also be other letters/numbers there to tell the date of manufacture, the brand/type of posi used, and which plant assembled the diff.

There is no one ratio that is best for quick take-offs and high speed. The lower(numerically) the ratio, the higher the top speed will be. The higher(numerically) the ratio, the quicker the take-off will be. Ya got to chose one in the middle somewhere! I would recommend staying in the 3:36-3:55 range, if you want to do any highway driving. The higher(numerically) the ratio is, the higher the rpms will be at cruizin speed. Adams' Apple38756.8392824074

How much differance is there between a 3.08, and a 3:36? IF I change the diffferential, should I change the POSI?

 

Thanks guys

You will notice a difference when taking off. It won't be like a Top Fuel launch, but it will be something you can feel in the seat-o-yer pants. I went from a 3.08 to a 3.36 in the last '72 I had, and it was well worth the effort of changing the housing out.
If you get a diff. assy w/gears, it should already have a posi unit in it.

I know it should have the posi in it, I was thinking of saving the extra like $300 and getting just the gears. But then I would need all the bearings, and races. If you were me what would you do? I don;t want to spend all that money.... I'm going to pamona in march to get some cheap parts, and I need all the money I can get my hands on. $1,200 is a lot for a DIFF assembly. $1,200 includes the core charge. Ecklers would probably charge me like $300 for shipping, their rates are retarted. Fast Corvette or Duntov Motor comapny $1,095 out the door, remanufactured... no core, free shipping.

Well, Louis, that's a loaded question! If I were you, I'd get the gears and do it myself, but.....I'm not you. I have the tools and knowledge to do these things. If you have the same, them by all means go for it! Sometimes it's easier for people to get a complete assembly and install it, sometimes it's better to do it yourself. Only you can determine this.
Diff. gears need to be set up very carefully in order to not be noisy, and survive. It is time consuming to get it right, and even more so if it's not right after you get it installed.
If $$ is the issue, why not stay with what you have, for the time being, and swap them later, when you can?

i know there exits a book with possible exchange parts from other GM cars.But i don't know if our C3 differentials where used in other models ?

Sometimes this can be helpfull as parts from the most expensive model seems to cost also the most as where the same part used on a cheaper model costs perhaps less...