Originally Posted by JCL

Twins:

A couple of comments.

You say that the pre x-drive has a locked torque split. That is technically true, but a little misleading IMO. The transfer case has a fixed torque split, but it is certainly not locked. The transfer case is open, and one drive shaft (front or rear) can rotate at a different speed than the other without too much issue. If you have a very large difference, for a long time, then damage can result. Towing with one axle raised and the other axle on the ground is an example; you are limited to a certain speed and distance by BMW.

Tire size differences can certainly cause damage with the pre x-drive transfer case. I would strive for 3-4 revolution per mile difference, but I don't think that is a hard cutoff. When BMW came out with the x-drive transfer case (in the X3) there were actually more problems with tire size differences. X3 models are famous for it, as they have a lighter transfer case than the E53. So I would be just as concerned with an x-drive transfer case and tire size differences, including on the E53/E70, probably more so than with the pre x-drive.