We have a rental car for one day out of Florence and would like a nice long hill town loop--is this do-able in one day (can leave very early and get back late)? Florence to San Gemignano to Volterra to Siena and back to Florence. And if so, is there a better order? It's hard to get a sense of driving times on maps and/or using bus schedules.
Try using viamichelin.com to get a sense for the driving times. I'm guessing that a 15 hour day would only give you about 3 hours in each town. You'll have to decide if that's doable for you. Probably good to leave the Siena to Florence drive for the evening return, as the other drives would be on local roads.