I just purchased a 2006 530i with sport suspension, thus it came with the 18" run flats. This is my first experience with run flats and I am wholly disappointed. My old E39 was so smooth, "supple yet firm" as BMW advertises. This one is bone jarringly firm. Can I replace the run flats with normal tires? I imagine I would have to purchase a donut spare. I loved the Kumho's I had on my E39. I don't enjoy driving this one near as much, yet.