3.15 95 m3
3.23 96-99 m3 manaul and 95 m3 auto
3.38 96-99 m3 auto
i went with a 3.23 because it is a big improvement over the 2.93 open diff and is very manegable for a daily driver. a 3.38 is supposed to be the best oem diff because it gives great acceleration and still has a decent top end. i would have bought a 3.38 but they are much more rare and command a premium price. when installing a new diff you will not need to have your speedo adjusted because there is a sensor in the diff that tells the car the proper speeds with the new gearing.
how would having closer ratio gears reduce the engines life?