Driving in Italy

It depends on the time at your disposal. If all you have is one day to drive, then you don’t have much time to visit much along the way. The driving time is 3 hours on a good day taking the freeway. If you take the more scenic secondary roads (like the Via Chiantigiana SR222 then the Via Cassia SS2) then it’s a much longer drive.

Is there a way you can borrow nights from the other destinations?
I’m not a fan of Milan, and I think one full day there is enough.
Venice for 3 nights is right and so is Florence for 3 nights. I wouldn’t bother for Pisa, but if you want a train can take you there from Florence in less than one hour. You can visit Pisa in an hour or two, there isn’t much to see beyond the Duomo, the baptistery and the leaning tower, all on the same piazza.
Rome deserves 4 nights, but if you have been there already, maybe you can do with 2 or 3 nights only.

If you can make the drive Florence to Rome last longer, like a 2 night trip along the way, you won’t regret it.

We stayed near Siena for 5 nights and drove all over Tuscany and Umbria. In the RS Florence and Tuscany guidebook, there are 3 driving tours that cover most of the area between Florence and the Val d’Orcia. You could spend a day on any of the tours. You can drop the car in Orvieto and take the train to Rome.

Since all of my lodging is already booked it would be hard to change Milan and Venice, and the small apartment I rented in Florence is already paid for. I chose two nights in Milan because I made a reservation for the Last Supper at 15:30 on 5/10, mainly because I didn't want to be jet lag stupid when I saw it. I would have to borrow a night from Rome, but since I'm booked into a small hotel between Termini and the Trevi Fountain (Hotel Aberdeen) I'd imagine they would expect me to pay for the night I don't stay there to keep the reservation for the other nights. In RS shows and movies I've seen the area I'd be driving through is beautiful and I thought it might be fun. I'm alone on this trip will little in the way of specific time commitments.

I would cancel the Rome hotel if they don’t want to move your dates. Generally most hotels allow free changes or cancellations if you do so at least 48 hours prior. If a hotel doesn’t allow, I stay away from that hotel. From what I gather your stay in Rome starts on 5/18, that is almost two weeks away.

Even if the Aberdeen is not available if you cancel, I see on Booking.com tons of hotels 3 or 4 star hotels available for May 20-22 under 150€/night in the city center of Rome. You don’t have to stay in the handful of hotels Rick Steves recommends. There are plenty that are even better.

If you leave Florence on the 17 May and arrive to Rome on the 19 or 20 May you have a few days to roam around Tuscany. You could even base yourself near Siena and take Day trips from there.

You don’t have time for Sorrento/Amalfi Coast. Let’s be serious now.

A friend who has never been there suggested that since I have no particular time to arrive in Rome that I might consider renting a car in Florence and take a leisurely drive

Since the friend has never been there, consider seriously the advice given here by Roberto and others who have been. The train is so nice from Firenze to Roma. You can always do a day trip to Orvieto from Roma if the big city fails to provide enough of interest. Then maybe plan a Tuscany trip for the future where you have a base camp in a hill town and a car for exploring for 4 or 5 days.

Unless you already have your IDP, you need to HUSTLE and get it if you even want to consider driving in Italy

EDIT---Oops-- missed that OP has the IDP

The last sentence of the OP says that Jim already has his IDP.

Jim,

Unless you intend to visit hill towns not well served by public transit, there's no reason to rent a car. I'd prefer to use the high speed train from Florence to Rome, and maximize my touring time rather than wasting time with a drive.

Note than any hill towns you might want to visit will have ZTL areas and possibly limited parking.

Pisa is an easy 1/2 day trip from Florence. It would be more efficient to do it while you're staying in Florence and not have the hassle of storing luggage in Pisa. There aren't that many train from Pisa to Rome and some require a train change. Trains from Florence take 1.15-1.5 hours, from Pisa, it's more like 2.5 hours. It seems that you'll have about the same travel time either way, so might as well do the trip without bags.

Taking a car for a day is likely to be expensive, and will be time-consuming to go through the paperwork to pick up and return the car, plus the time to get from your hotel with your bags to the rental agency in Florence, then from the agency in Rome to your hotel, not to mention the time and stress of driving in Rome itself.

I picked up a car in Florence, drove to southern Tuscany where I stayed for a few days, then drove to Rome FCO where I dumped the car. Florence to Rome is doable in a day but not if you plan on taking scenic routes or stopping to look at stuff, and there is a lot of scenic stuff on the way. Unless there is one particular thing you have to see, might be best to stick to trains. I haven't seen Orvieto but I think it could be done as a stop by train on the way to Rome. Others can comment on that option.

I did like Milan more than most on the forum. If you're into art and city life, there's lots to keep you busy.