I think the tires will be fine without TPMS Sensors, the weight of the car isn't that heavy an should allow you to drive off the road. You should be able to feel the tire being low, typically more vibration or sloppy steering. I currently run Lorinser Speedys, but at the first sign of snow I am putting on my stock rims without sensors. I saw that Tirerack removed their Pirelli Snowtire offering for Smart Cars. I drove the Smart this past January in light snow and it did fine with the stock tires.