Comments by "Golden Croc" (@GoldenCroc) on "Drag-Built 500 AWHP BMW M140i xDrive on the Nürburgring" video.

@jhill92 Its not the width, its the diameter. For instance 225/40 and 245/35 are very close in diameter. You should always have the same diameter within a few percent front and rear on an AWD car, yes, even a car such as yours or the computer controlling the awd diff might get mixed up and wear it out and other not desirable effects. Width and permanent AWD has nothing to do with it. For your car 245 35 and 265 35 should be ok, but I would not run more difference than that.
3
@jhill92 Quite right, but since you wrote "bad for the xdrive to have different width tires" I decided to reply, Its a common misconception after all. Another common misconception is that the aspect ratio number on the tire is not a percetage of the width, but some other number, such as millimeter height. Just looking out for fellow enthusiasts :)
2