You opened this can so here goes... you say to get a tire with a decent sidewall, there are only a few tires with the recommended size for a smart car. to use any other size will affect possibly handling, stability and fuel mileage. I find it fascinating when people try to second guess the Mercedes engineers that designed the car. They take a lot into consideration and if you are willing to make sacrifices in some areas to better utilize others, then go for it. (i.e. lift kit for more ground clearance, larger tires or changing springs for special driving needs)
Run flat tires are specifically made with an inner construction to handle the load if the tire looses air pressure. they weigh much more than a regular tire, and are much more expensive.
Self-supporting run-flat tires are now common on light trucks and passenger cars and typically provide for the vehicle to drive for 50 miles (80 km) at around 50 miles per hour (80 km/h). However, if the tires are subject to this kind of misuse, they may become irreparably damaged in the process. In addition, if the tire is punctured in the sidewall or at the edge of the tread, repair may be impossible or unsafe. These tires carry a 20 to 40 percent weight penalty over similar standard tires. The thicker sidewall also means higher rolling resistance, thus reducing the vehicle's gas mileage.
Had these on my 1962 Rambler station wagon. You had to inflate the inner section with a special tool and the outer section like a regular tire. If it did go flat and you used the run flat feature, it would destroy the tire and was mighty expensive to replace. they also rode much harder than a normal tire couldn't wait to wear them out and put some normal tires on there. I'm sure the newer tires are much better but they are not available for smarts.
For the last 4 years, and 102,000 miles I have only had one tire pick up a screw that I noticed before it went flat. Newer tires don't have the same problems as in years past.
