Have replaced four tires on my 2017 M240ix in as many years potholes on local interstate highways from (each a single tire blown, but I don't choose replace single tires). If I invest in replacing the 225/35/18's with 235/40/17's, will I buy added protection? How much violence will that do to handling? Walt DC area
Going from 18 to 17 nets you an additional 1/2-inch of tire sidewall if your overall diameter stays nearly the same, (which it does in this case, from 24.2" to 24.4") so that's a minor additional hedge against blowouts & rim damage. Not sure what you mean by violence to the handling - going to a slightly taller tire, at it's handling limit, the tire will be rolling-over more, comparatively, and turn-in response wouldn't be quite as sharp. I think on the street, that little bit of difference would hardly be noticeable in any normal driving. That's the kind of thing more likely noticed on the track pushing the handling limits and performance envelope of the tires. Probably less-noticeable difference if you're on runflats and stick to runflats for the new size, with their stiffened sidewall.