Here's the info on the rim for the 540i that you are trying to use on the X5.
X5 E53 = PCD 5x120, centre bore 72.5mm and M14x1.5 bolt thread. ET 40-45
5 Series E39 = PCD 5x120, centre bore 74.0mm and M12x1.5 bolt thread. ET 15-25
You can see that the centre bore for the X5 is only 72.5mm and the center bore for the E39 is 74.0mm. So there is a difference of 1.5mm, or if the wheel was perfectly center a gap of .75mm around the hub.
Also the holes on your rims are smaller than the lug bolts that are used on the X5.
Also the higher the ET number, the more the rim will sit inside the fender. So with the lower ET number on the E39 rims, the rims sit further outside the fender. That's why the wheel look wider.
Last edited by upallnight; 01-30-2013 at 04:21 PM.