Do I ditch the rental car in Tuscany, or drive it to Rome so we can explore along the way?

Tolls mean that you have to pay to use them. The A1 is a toll freeway, the Aurelia along the coast is not.
Whatever route you take I would drop it upon arrival in Rome. The route you take depends on your preference and what you want to visit. If you want to stop in Orvieto or Civita, take the A1. If you would rather visit Mt Argentario or Capalbio, then you would take the coastal route. The A1 is faster, if time is of the essence.

I wouldn’t bother with dropping the car in Siena, too much hassle to get to Rome from Siena. If you absolutely want to drop it closer to Siena, do so in Chiusi, then you can take a train to Rome from there. If your rental is from Hertz, you can also drop it a bit further south in Orvieto.

If you are renting a Hertz car, turn it in across the street from the Orvieto train station. It is maybe 90 minute drive southeast of Siena and a 70 minute train ride into Rome Termini.

Thanks for the great information, do you know if it is necessary to buy a GPS from home (California) or can you just use your iPhone, and this does sound funny, but we have Verizon, do they have the same carriers in Italy? We are thinking we'll only have the car for 3 or 4 days so a GPS won't be necessary if we have our phones but not sure of wifi near Tuscany area. Thanks for you help.

Google maps app on your phone will work.
Cell phone providers in Europe are not the same as in the US.
There will be international data roaming charges charged by Verizon. Inquire on their international rates.
There is also the possibility of installing a local SIM card in your phone, but that is a whole new topic.

do you know if it is necessary to buy a GPS from home (California) or can you just use your iPhone, and this does sound funny, but we have Verizon, do they have the same carriers in Italy?

We took our Garmin with an Italy/Greece maps SD card and we also took our Verizon iPhones and iPads.

I think is six of one, half-dozen of another. The Garmin did a great job and was very accurate. It also displayed speed limits, which was helpful. We did try our phones and they did a good job as well.

The Garmin SD card is $40. Our Verizon international plan is $10 a day for roaming and it uses your phone's data plan. I you used Google maps every day, it would be $40.

The advantage of the phone is there is a one time charge of $10 per 24 hours, so you could use it for other things as well. However, almost everywhere we went had free w-fi. The disadvantage would be coverage, but I can't recall having any problems with coverage.

The big advantage of the Garmin is not having to worry about cell phone coverage. Another advantage is the ability to easily store routes. I actually saved several routes I knew we would be taking before we left for our trip. As soon as we pulled out of FCO to drive to Tuscany, I pulled up the saved route to our agriturismo and the Garmin took us straight to their front door.

We took our Garmin with an Italy/Greece maps SD card and we also took our Verizon iPhones and iPads.
I think is six of one, half-dozen of another. The Garmin did a great job and was very accurate. It also displayed speed limits, which was helpful. We did try our phones and they did a good job as well.
The Garmin SD card is $40. Our Verizon international plan is $10 a day for roaming and it uses your phone's data plan. I you used Google maps every day, it would be $40.
The advantage of the phone is there is a one time charge of $10 per 24 hours, so you could use it for other things as well. However, almost everywhere we went had free w-fi. The disadvantage would be coverage, but I can't recall having any problems with coverage.
The big advantage of the Garmin is not having to worry about cell phone coverage. Another advantage is the ability to easily store routes. I actually saved several routes I knew we would be taking before we left for our trip. As soon as we pulled out of FCO to drive to Tuscany, I pulled up the saved route to our agriturismo and the Garmin took us straight to their front door.

How was the coverage of the Italy maps on the Garmin? Is it up to date?

I looked at the Garmin website and they sell the SD card version for $70.

How was the coverage of the Italy maps on the Garmin? Is it up to date?

I looked at the Garmin website and they sell the SD card version for $70.

I bought the card from Amazon for $40. I just checked and it is now $49 at Amazon.

I didn't have any problems with the maps not being up to date, but you make a good point. It would be helpful if you could find out the last time the maps were updated. I'm sure Google maps are more up to date, but unless you are traveling to an area where you know there a road changes being made, I don't think you'd have a problem.

Entertainment is a side benefit. We loved listening to Miss Garmin pronounce Italian street names!

If your phone is unlocked or can be unlocked, I would just purchase a SIM card in Italy. (Check out the “Telephone & Tech” section of Travel Tips on this site for more info.) I’ve used Apple and Google Maps and they worked well. My favorite app to use is Waze because it alerts you to traffic conditions and warns you about speed cameras and traffic tutor zones. Familiarize yourself with ZTL’s along your route and make sure you get the mandatory International Driver Permit from AAA. If you want to check on ZTL’s, here’s a very handy website https://www.accessibilitacentristorici.it/Default.cfm?lingua=EN

Years back Garmin offered units with both North America and Europe Maps. Now you’d have to buy a GPS and then pay extra to get the Italy or Europe map.

Thanks for all the great information, when we return in May I will update all about the coverage and what we end using and how well it worked.