PS: If you don't want to read the long story, just skip it and go to the bolted and underline sentences.
I have an E31 with a 6sp gear box and 2.65 final drive. A source told me that the E31 differential is interchangeable with E32 and E34. I want to change the differential since I think that it is way too tall now. I'm driving the car at 4th gear at 3000rpm at 80 mph. What a waste of the power band. A friend suggests me to change it with 3.64 from the 850iA. However, I would like to double check about what is the differential ratio for the E34 M5 with the 6 sp transmission? What ratio do people use to upgrade their E34 M5 with 6 sp?
'91 850i 6 sp
'00 Racing Dynamics R50 Sedan (E39 based) 6sp