I agree - if you have a B18, you most likely have a 4.10:1 ratio, but others are possible. Dana 27 gears come in a variety of ratios all the way down to 3.07:1.

Even if there is still a tag attached to the diferential, it may not be corrrect, although it most likely is right.

The only SURE way is to open it up, and do the tooth-count thing. You need to inspect it and change the lubricant anyway, don't you? You may as well plan a weekend job that should take only an hour or so to jack up the car, take off the cover, count the teeth while the lube is draining, and put it back together.

If you damage the gasket, they're available at any parts house. You may want to pick one up before you open the cover, although it's fairly rare to damage that gaset unless you get wild using a pry bar to get the cover off!

I put LUCAS diff lube in mine when I serviced it last, and it's much less noisy now. You'll want to pick up a little hand-operated lube pump to fill the differential instead of trying to use the squeeze bottle it comes in. Much less messy and you can use it for a lot of things. They should cost about $5.

Have fun!