I recently put 4.8iS brakes on my 3.0i, I am currently running the 18 inch wheels. I have no clearance issues. (although I had to file a tiny bit off my wheel weights to get them to clear! ) I am not using any wheel spacers either. One thing I did notice, is that I need a new spare! as it seems the 3.0i all came with a small 17" spare tire/wheel, meaning if I get a flat up front, I have to move the rear wheel to the front, and put the 17" spare out back! not ideal to say the least!!!

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2003 X5 3.0i (02/2003) sport package, Sapphire black metallic / Black Dakota Sport seats. compass mirror retrofit, trailer hitch retrofit, Prodigy P2 trailer brake controller installed, PDC retrofit, Lumbar support retrofit. heated rear seats retrofitted. Running boards retrofitted and subwoofer install in progress. 4.8is Brakes installed front and back. Bluetooth TCU installed, Rear Fog lights installed. Retrofitted reclining rear seat backs.