Where to rent a car in Rome when driving through Tuscany for a few days to Florence

Many use the airports, particularly if they are nervous about ZTL or about securing an automatic. Some take a train to the next sizable town (out even to an outskirt of the city in question) and pick up there. It really just depends on your needs and wishes. A Florence drop off does not seem to be too problematic.

Why are you driving to Florence? Merely a question.

Traffic Fines are stiff in Italian cities and parking difficult.

Airport pick up will be more expensive but you will probably have more options as far as type of car

You could take train to Chiusi and pick car up there, could also drop car there then train to Florence

Be sure to note opening hours of wherever you pick up
Most smaller locations will close for a few hours mid day, by noon on Saturday and closed all day Sunday

Go to www.AutoEurope.com and see what your options are
Rental rates are sky high this year so only get the car for the days you really need it

Here is a recent post with advice and be sure to heed info in last post by Nigel
https://community.ricksteves.com/travel-forum/italy/car-rental-when-leaving-florence-need-advice-please

We want a rental car for 2 days to enjoy Montepulciano and the plan was to return the car at the Florence airport when we arrive there, and then not have a car while in Florence.

Going to Rome Fiumicino airport to pick up the car is not necessary and more expensive, considering you are already downtown Rome. Airport fees will add to the rental cost, plus you will need at least 50 euro to get to the airport by taxi.

There are several options in Rome downtown, all of which pretty easy to get to the freeway. I will send you the info later.
Regarding returning the car, a good options would be to return the car in Chiusi (rental offices are near the station) and then take a train to downtown Florence. You could even go to Chiusi by train and pick up the car there. If you need an automatic car I'm not sure about the availability there. Chiusi rental car offices are not large and generally automatic cars are available only at locations where Americans pick up the car, like downtown Rome or at airports. Larger vehicles are more likely to have auto transmission. Economy cars rarely come with auto transmission.

I'll send the info separately for returning the car in Florence. Returning at the Florence airport is not the best options in my opinion. The Florence airport is on the opposite side of town from where you will be coming from and you will have to spend 30 euro to go downtown by taxi. I'll send you the Florence return options separately.

The stretch from Rome to Florence is fabulous. Think about a stop for Cervetari and Tarquinia just north of Rome for the wonderful Etruscan tombs. Or Sovana where you can pick up a map and then drive up into the hills to see hillside tombs. Montepulciano is wonderful and the central hotel has parking -- and there are a dozen good restaurants. If you can stretch this part of the trip to a couple more days -- 4 rather than 2, it will repay the effort.

We with a similar agenda a few years ago picked up the car at the Airport and drove to Montepulciano for two weeks where we then toured Tuscan towns, abbeys etc and then we dropped it at the Florence airport which avoids the really sneaky and lethal ZTLs of FLorence. It is trick to drive to the downtown car rentals without racking up tickets -- just takes one aimless confused turn around the block to get 2 or 3 tickets.

There are also rental agencies on the edge of Rome which might work -- we did that decades ago but it was before ZTLs in Rome.

ROME CAR PICK UP:
TERMINI STATION:
https://goo.gl/maps/6t15wcH5BPapSH2G8
The place above is for Hertz, but all other rental car agencies are also in the same place (Europcar, Avis, Budget, etc.)

TIBURTINA STATION (from this location it's even easier to get to the freeway). All the companies below are across the street from the station, just a few feet from one another.
Hertz
https://goo.gl/maps/ihXnhmfgYWqFJJbE9
Europcar
https://goo.gl/maps/17PwUnM8Zmf7w3jT9
Avis Budget:
https://goo.gl/maps/K1G522z3rdB5KT48A

DROP OFF FLORENCE
Hertz Firenze Sud (this is the easiest coming from Montepulciano)
https://goo.gl/maps/WJbAU1wYwC7ADw84A

Interpark Garage Europa (downtown) - this is car rental return downtown (near train station) for all companies:
https://goo.gl/maps/Ke2HmaBfGwiLF8ty6

Hertz also has another location that is easy to reach, on Via del Sansovino. However it is temporarily closed (I think it might reopen next July).
https://goo.gl/maps/f1smoeQPRXWkQdbo9

Use Google Maps for directions. The only location near a ZTL is the Interpark Garage Europa in Florence. However if you follow Google Maps it will steer you the right way without encountering any problems.

Having said that, I think the easiest car rental return would be in Chiusi (near Montepulciano), and then take the train to Florence. I would choose this option unless you want to visit something on the way from Montepulciano to the car rental return in Florence, or unless you have severe mobility problems that would prevent you from getting aboard a train (there are sometimes a couple of steps to get on board). Chiusi has Avis and also Europcar. I'm not sure if Hertz still has it (I think they closed during the pandemic).