I need to know the rear end gear ratio for a 1980 firebird - saf-t-track  

take the cover off and look at the numbers on the ring and pinion
or do the (turn  d shaft,and count the times it takes to make 1 complete turn on the rear wheel)
if its in the factory car check the vin in glove box/ tag
look for the posi tag  

Common stock gears for 80 are 2.41, 2.56, 2.73, 3.08, 3.23. As Jason12 says, pull the diff cover and rotate the ring gear while wiping the oil off the edge, the numbers you are looking for are a pair of two digit numbers (ie 12-41) divide the larger # by the smaller # and you have the ratio. If it is a Saf-T-track you can tell by jacking up both rear wheels and try turning the wheels with the trans in park (or in gear if manual trans), the wheels shouldn't turn, (if it is an open diff, the wheels will turn opposite directions). Saf-t-track should also have a stamped metal tag that says "limited slip" (chevy calls it Positraction- same thing).
Hope we helped.