Most small towns have closed their historical centers to cars, so you need to find parking outside of the center and walk around. San Gimignano for sure does not allow cars inside the walls! As soon as you arrive into the small towns, look for the "P" signs that indicate where there are parking lots to leave your car. The centers that restrict traffic are called "ZTL", zones that limit traffic. You never want to keep driving into any center that says it has a ZTL (like Florence, Siena, Pisa, Lucca, etc) because you will receive fines later on, no matter where you are. The local government has cameras installed that take photos of all cars and check them against those that have permits to enter the ZTL area and rental car companies give out your info, since they know who the car was rented to at a certain time on a specific date. So you might get a fine months or years after the violation, and it will ruin the memories of your trip. So don't plan on driving in any of the small towns, park outside and walk - some are small and the walks are short!