easy,quick,down and dirty way is to jack it up...mark the drive shaft
with chalk or a piece of tape...grab a rear tire rotate it one
complete time...and count the number of times the chalk mark or tape
rotates...  slightly more then 3 times = 3.08 etc... slightly less =
2.93   etc....

Bob Griffiths

Or you could do 1/4 of the work that Bob suggested: jack it up and look
for the stamped in code on the bottom of the differential flange.
According to Chevylay your '77 ought to have one of the following:

OA - 3.08
LR-OD - 3.36
OB - 3.55
OC - 3.70

Here's waving to ya - \||||

Owen
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'67BB & '72BB
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"To know the world intimately is the beginning of caring."
    -- Ann Hayman Zwinger