what is the most accurate way of calculating a diff ratio. would it be to turn one flange and see how many times the other flange turns or is there another way

i take it its sierra the only real accurate way is to take the back off the diff and the ratio will be stamped on the crown wheel. dont go by the tags these have been changed in the past by some less honest people.

should be a tag on the rear of diff!!

How much difference does it actually make? I mean could you really tell the difference between a 3.18 and a 3.88 just by driving it if everything else on the car was the same?

a hell of a lot.
yes
3.14 = 134mph approx in the blade
3.92 = 108mph approx in the blade

Ah ok then, so lower number means faster top end too...

I think that's why people settle for a mid-range ratio, as with everything there are +'s and -'s the greater the top end speed the slower it will pull away but lower top end great for drag racing at lights

tony