tallest/longest diff that fits the E30?

[SLOWER]

pretty much, its gonna be the slowest thing on earth. but thats what I need. im planning on doing suspension. itll still be fun in the twisties and save me some waffleswaffleswaffles at the same time.

[chadthestampede]

the most boring e30 you mean.

[SLOWER]

Im working on the most ultimate gasoline saving utilitarian e30 i can make, and so I want to throw the tallest diff in there I can. Ive heard numerous 5 series may have what I am looking for but I have little as far as specific info.