[tcbrown76]

I also have a 340i. I haven't changed by runflat stock summer 19"s so I can't compare the difference, but I don't mind them. I do, however, have non-run flats for my winters and there is a significant difference in ride feel (not as tight or stable with the winters than the summer tires). Agreed with above posts that the bigger difference in ride is Sport mode vs Comfort mode, though.

I have an entire winter set up on 18" rims and they fit fine on the blue M brakes. That said, I am not certain that ALL 18" rims will fit, so you should be careful before you make a purchase to measure or mount the wheel and see if it fits. Though, I would venture to say that most 18" rims will fit.

Good luck!