2.24/2.47/2.88 (you have 3.07 now). Only 85 and before diffs work. The 2.88 is in the '85 300d/300sd, the 2.46 is in the gasser W126s, the 2.24 is in the W126 coupe.

You're only looking at a couple mpg increase btw. Also, drive slower, it's not just rpms but air resistance too.