On a whim I looked into other differentials for the e70 family. It appears the xdrive50i has 3.15 ratios as opposed to the xdrive35d 3.64 ratio. Anyone speculate as to whether these would bolt up onto the diesel? Would probably drop about 325 rpm on the diesel at cruising speed. Would put 70 mph at 1730 rpm, 80mph at 2030 rpm, which I think would make a nice difference.

Not sure if the front diff would have to also be changed or x drive doesn't care?

I was thinking about the same project. 3.15 diff are pretty cheap. I think the 50i diff should work in a diesel. The 35i also got 3.15 w/ the 8 speed but the axle shafts are smaller so the dif is different. 50i and 35d use the same bigger axle shafts. F15 diesels w/ 8 speeds also use the 50i diff.

6hp 6th 0.69
8hp 8th 0.66 or 0.64
From wikipedia of each model.

So yeah the f15 with 8hp top gear isn't that much taller but the diff drops their rpm a lot!

Do they all run the same front diff ratio? Or does that have to be matched for the transfer case xdrive???

Front and rear diff ratio must match.