Recently I've been delving into the idea of new gears on my SS M6 until I realized, I really had no idea what gears would be best or how they even really worked. I used the search to pull up a bunch of threads that I sifted through for info for hours and I realized a lot of other people seem to be confused about gear function too!
Anyway I stumbled on this video by accident and although it is a bit archaic, it's actually pretty informative check it out,
Skip to 1:50 though
oh and I think 3.91 gears are what I'll end up going with!