I prefer replacing the 4 for an ideal balanced grip. I don't have the guts to have the rear and front axles of different grip... maybe i am just imagining stuff, but i have a feeling that if you enter a fast curve on a wet road for example, chances are much higher to go over/under steer in case you have different tires front/rear (old vs new)

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BMW X5 3.0si 2008
Premium Package
Light Package
Navigation
Reverse Camera
Rear Sensor (retro-fit)
Comfort Access
Spare Wheel
Power folding mirrors
Voice Control w Bluetooth
Power trunklid

BMW X3 3.0i 2006
M Aerokit
M Sport Package
Navigation System
BT Handsfree
Xenon HID

BMW X5 3.0i 2001 --- SOLD