on a set of 17's running 235-45R17's will have a total radius from the centre of the wheel to the circumfrence of the tyre of 322mm, which means that you only need 8mm clearance from the tyre to the guard. Not much really and there wouldnt be many cars running lower than that. Just as a point of interest, 15's running 215-65R15's are less than 5mm different in overall diameter to the 17's