How can you tell what rearend is in the car?

I have a '79 Firebird Esprit it came w/ the buick 6 cylinder. The rearend housing is all rusted and I don't see the tag. Is there another why to tell what's in it?  

70-81 firebirds had 8.5 inch ring gears (10 bolts on cover) usually there are two letters stamped into the drivers side axle tube, looking from the rear of the car.  

Typically non-posi rears didn't get a tag so you have three choices: 1-locate the code mentioned above, 2-pull the rar cover and look for numbers stamped on the edge of the ring gear next to the carrier (ie. 12 & 41) divide the larger number by the smaller number and you get the ratio (3.41:1 in my example) or 3-jack up the rear wheels and support on stands, mark the tires and the driveshaft and turn both tires the same direction while someone counts the number of turns of the driveshaft. This will get you a close idea (almost 3 and 1/2 turns= 3.41). Also finding the build sheet, often between the rear seat back and the trunk divider cardboard, will tell you what the car was built with. In 79 Firebirds were available with 2.41, 2.56, 2.73. 3.08 and 3.23 gears although there may have been 3.42 available as well. I would bet on your car having the 2 series gears.
Steve  
Trying to help... 'cause we don't all have to learn the hard way!

Thanks for the help, Steve.
could you tell me if any of those ratios would be good for a 400 w/ a 700R4 combo?
or should I go bigger like 3.73 or so? I'm not racing this car just gonna be a driver.(although fast at times)  

A 3.23 or 3.42 gear would work well with the 400 if it is a true Pontiac engine.  
professional hi-performance engine builder

Horsepower sells engines and torque wins races

As Pontiacman8 said, these gears will work well with a true Pontiac V8, Chevys tend to like deeper gears. Do you already have the engine/trans or just thinking about it? Personally I like the 200-4r better than a 700r4 simply for the ease of installation and the better gear ratios and they have a proven track record behind stout Pontiac v8's as well.
Steve  
Trying to help... 'cause we don't all have to learn the hard way!

Yes, I have a true 73' pontiac 400, and I already have a 700R4.  

Hey guys, don't you need somekind of adapter to fit a 700r4 to a pontiac block.  I thought they only came with a chevy bellhousing bolt pattern and did not fit.  

Yes you need a adapter to bolt the 700r4 trans. to a b.o.p. engine and can get them from jegs.  
professional hi-performance engine builder

Horsepower sells engines and torque wins races

There is a code stamped into the passenger side axle tube facing forward near the middle. You may have to scrape off some grime to see it. Both my 75 birds had it. I think it gives the build date and ratio. I don't recall if I had to look someplace else to decode though.

After posting I found this, http://www.wallaceracing.com/axledata2.htm
you do have to decode it. Unfortunately it only goes to 1975.  

I found this while digging around, I haven't had a chance to crawl under my car yet to verify, but according to this site and my VIN I have a 2.41 posi. does that sound about right? and will it be ok for my engine/trans.


http://www.transamtech.com/79_tech.shtml  

If in fact you have a 2.41 gear then I would go with a 3.23 or 3.42.  
professional hi-performance engine builder

Horsepower sells engines and torque wins races

ok, thanks everyone for your help.  

No problem.  
professional hi-performance engine builder

Horsepower sells engines and torque wins races

I just crawled under my car today and found the stamp on the axle, It starts with
2 PH, the PH means it's a 2.56, but what does the 2 mean? if anything