I bought a 2017 M240i convertible with about 4000 miles (it had been a dealer loaner car before I bought it). The ride is harsh. The dealer's head service guy says it's normal; they checked tire pressures, and no shipping block problem. Present tires are runflat Pirelli Cinturato P7 225/40 R18. Would switching to non-runflats and/or another size improve the ride? Any drawbacks except $ and lack of a spare tire?