Differential Upgrade

I suggest that you tell people why you want to change diff ratios? Are you after more top end or more grunt at the bottom? I am assuming that this is an e46? How far are you willing to go? Are you after a 3rd party housing? The quick answer, if you are after marginally lower gears from a bone yard is to throw in a unit from an M or a ZHP (a.k.a - M Performace Package), which is 3.07. Having said this, why not just go with the 6 speed in the first place? You're already half way there. By the way, I have a ZHP and I still can't use 6th reasonably without incurring the (unwanted) attention of the County Mounties ... unless I'm happy cruising at 120 KPH and ~2000 RPM. Suppose if I were cruisin' down the Autobahn, ...but over there I only get 4 bangers, ... Sucks to be a German stranded here in North America!

Better get an LSD diff with a 3.07 or 3.46 ... #.07 is better if you have a plan to FI your car later ... 3.46 if you wanna go NA all the way