Lots of people do it all the time. I'm sure you know you cannot drive into the city so you would need to find parking. You will have the benefit of being able to stop at other small towns but you run the risk of fines for driving where you're not allowed or for speeding. Taking the train is cheap and easy though more restrictive.
With only a day to spend in Florence, a car will be more of a nuisance than a help. The train is efficient and everything that you'll probably want to see is well within walking distance. To maximize your time there: leave as early as you can; definitely do some research so that you can "plan" your itinerary, and consider making advance reservations for any museums (ie. the Uffizi Gallery) so that you can avoid waiting in line. Visiting the Duomo is a given, but I also highly recommend Santa Maria della Croce and its opera (museum) - and - a great nearby restaurant: Osteria de Benci on Borgo Santa Croce 31r for a "feast" of pate, "drunken" spaghetti and Chiannina beef. You can sleep it off on the train ride home!!!!Edited: 10 July 2018, 19:45
I would go by train, not by car but it is a personal preference of course.
For a daytrip I would not go to the Uffizi unless you are really art lovers. But if you want to go, yes make reservations in advance otherwise you will stand in line for a very long time.
With one day, explore the center on foot and then either walk up or take a bus to Piazza Michelangelo to admire Florence from above. In the Florence forum I think there are nice one day iteneraries.
I agree that train is basically better and that hiring a car might be a nuisance more than a benefit. You'd have rather short time to spend in Florence, so no use "wasting" it going by car and/or stopping here and there.
"For a daytrip I would not go to the Uffizi unless you are really art lovers"
I tend to agree but I would say: I wouldn't go to the Uffizi unless you have some specific interests in THIS museum. If your interest is generally "art", you'll find in Florence countless alternatives, easier, cheaper and more practical to visit.
That is what I meant to say Aristofane. You explain it so much better than me.
The Uffizi was only given as an example of one of several museums that tend to have long lines; the point is to do one's "homework" if time is limited. I don't assume "love of art", but usually if someone is going to Florence for one day and they've never been before, I do assume that there'll be special things that they'd like to see.
How kind of all to give me some great direction. After discussion with my group, we have decided to drive to the Hilton Florence by the airport, catch a cab and enjoy the entire day in Florence. After a long day we will simply spend the night and then drive back to Orvieto. Thanks again for your suggestions!
It's assumed the OP already has a car.