Florence - Taxi to the Airport early in the AM

Can you catch the train?... it would seem to be just as fast when factoring in the airport hassles.

Mike (above) makes a good point: Have you considered taking the train from Florence to Rome? That's how most people do it. The fastest trains get you from Florence city center to Rome city center in 1hr 35mins, station to station time.

Maybe you're connecting to a departing flight out of FCO? If not, consider saving time and hassles with the train.

I haven't tried returning a rental car in Florence (after making the mistake once, I have a policy of not having a rental car in Florence)--but if you stick with your plan of flying from Florence to Rome, then probably better to return your rental car the day before; and then ask your hotel to arrange for a taxi to pick you up at a sufficiently early time.

This simplifies things and eliminates a couple of complications that could occur if you try to return the car the morning of the flight: eliminates the possibility of route-finding problemos when you're under time pressure; and eliminates an additional possible problem with returning the car the morning of the flight is that, unlike here, where 24-hour return exists at most airports, sometimes in Italy opening/closing times will blind-side you, especially early on Sunday mornings.

Hopefully you already know about the Florence ZTL's and won't return the car to a location that will require you to go into the ZTL'a.

I was with a friend in Florence and he had the same problem, we even stayed near the train station, on a sun! He asked the hotel the night before if they could arrange for a cab at 5 am. When he got up the hotel made the arrangement and everything worked out fine.