Rental car to leave and return to downtown Florence

Mary, look for Roberto's comments. He used to live there. He should be able to provide you with really good instructions on how to exit and re-enter Florence without intruding into the ZTLs.

I have, every year (from Avis, on the same street). You have nothing to worry about. Borgognissanti is a Free zone corridor within the ZTL (limited traffic zone). Hertz will give you a map where they will highlight the itinerary to get out and return safely. Borgognissanti is one way street (east to west). To get to ss222 you have to go to Viale Europa. Then from Viale Europa you must go to Ponte a Ema. Become familiar with the itinerary on google maps.
I'll give you the itinerary when I am at a real computer. Mobile devices take to long to type.

Thanks so much. You have already relieved some of my stress on this issue. Waiting to hear more details

Using Google Maps (Get Directions) use the following: From (A): Borgo Ognissanti, 137, Florence, Province of Florence, Italy To (B): Grassina, Province of Florence, Italy Google Maps will yield more than one itinerary. Choose the one that takes the routes SS67 and SS222 (SS stands for Strada Statale, i.e. State Road). That is the easiest. Print that itinerary. Grassina is a town on SS222 that you have to go through. If you reach Grassina, you can't get lost anymore. Just follow the main road, you are on the Chianti Way, or Via Chiantigiana, (aka SS222). If for any reason you get lost in the city. Just ask for Grassina (pronounce GRAssina) emphasis on the first syllable. Grassina is your beacon. The route will take you on the boulevards that circle the Old City of Florence (which is all traffic free). Basically from Borgo Ognissanti you go straight to Piazza della Porta al Prato (there is a Old Wall Gate tower there), where you turn right and enter the boulevards and follow the boulevards all around (see the map on Google Map).
Once you go around the historical center on the boulevards, you'll find yourself along the river. You follow the river upstream on the northern bank until you enter the freeway and cross the river. That turn is tricky. That bridge (the Varlungo bridge) is a double decker bridge. You go on the upper level. Follow that freeway but keep an eye for the exit to GREVE-SIENA (SS222). That exit is just before the freeway toll booths. Don't go to the booths (that's for Bologna or Rome). Exit at the last exit before those booths. You'll be passing next to the Sheraton. Keep going, always keep an eye for the GREVE-SIENA (SS222) blue sign. When you enter GRASSINA give yourself a pat on the shoulder. You've made it.

Thank you again, Roberto. I have read many reports today that if we have to return the car at the same place, there is almost a certainty that we will get a ticket. These are from people that rented from Hertz. It is much harder returning than leaving town. So I am considering changing or dropping that rental and leaving from the airport. That means a walk to the train station and a bus ride to the airport but better than getting tickets.

I return my car in Borgo Ognissanti all the time. Never gotten a ticket. The agency will give you the itinerary to return. It's not that difficult. There are now red lights before you enter the ZTL. You'll see when you are about to enter a ZTL gate.