A car is a liability in Florence and the horror stories about trying to navigate the central city are all true. If you want to enjoy a relaxing vacation, you really don't want to drive in Florence.

If you want to hit some hilltowns on the way to Sienna, look at going to the Florence airport at the end of your Florence stay and rent a car there, Or take a bus to Greve and rent there. Either way, you avoid the horror story.