Where to park outside of Rome and then take transportation in and walk around
Where are you coming from?
driving from Florence
It can matter where you're coming from, but one of the easiest garages to get to is Mercato Trionfale Musei Vaticani. It's located under the Mercato Trionfale (market) about two blocks from the Vatican. Because Prati is a newer part of Rome, the streets here are wider - and make more sense. I lived about 75 meters from this market and all our visiting friends parked here.
It used to cost about 1.50€/hour and no more than 20€ a day. If you're comfortable riding buses, walk 70 meters to Via Candia, 111 - in front of my former apartment - and catch the 492 bus downtown. It stops right at Piazza Navona and many other locations, including Largo Argentina. (you can ride the 492 back within 70-80 meters of the garage).
You could walk down the block and catch the Metro at Ottaviano ... or simply walk - about 20 minutes - to the center of town. A great walk is to head through St Peter's Square - another country - and then by Castel Sant'Angelo, over to Piazza Navona...and then head towards the center! We often walked this route to dinner each night. If you get tired, ride the Metro back to Ottaviano or Cipro and walk a short walk to the garage.
Because the Prati area is newer, the roads are laid out more in a grid... but you will still find many one-way streets. Coming from Florence, you can get off the GRA at SS2 (Via Cassia) and come right into Prati. Again, if some of my friends could find and use this garage, I have no doubt you can. Be forewarned, driving anywhere in Rome WILL be an experience!!!
I would like to park about 15-20 minutes from Rome
It would be simpler, quicker, cheaper and less stressful to take the train from Florence to Rome
City center to city center
Not sure why you’ll have a car in Florence?
The fast train, "freccia" types, go Florence to Rome in 1'36". Same on the way back. This is much faster than driving!