Italy By Car

If you really want to do this, you have the right idea. Rent a car as you leave Rome; use the car to see everything up to Siena; return car in Siena; take express bus from Siena to Florence; take train from Florence to Venice. No need to keep the car while in Siena, and it will only cause problems with potentially straying into the ZTL or limiting which hotel you use in order to stay outside the ZTL.

Whether this is the way to use 8 days in Italy is a different question. For me, it's about twice as many places as I'd want to see, even in a rushed itinerary. You'll be changing hotels almost every night - not fun. And you've budgeted 1.5 days for Rome, but the first day is often a jet-lagged haze. But it's your trip.

For car rentals, I found Kemwel had far better rates than AutoEurope or Gemut or the rental agencies themselves, but you should check them all to compare.

For hotel recommendations, what is your budget (in euros)?

It is an awesome itinerary for 18 days though...

How many nights in each place?
Let's say a min of 3 nights in Rome, 2 in Florence, 2 in Venice. Where is the time for the rest?

Take a 70 minute train ride up to Orvieto to pickup the car. The Hertz office is across the street from the train station.
It's maybe a 90 minute drive up to Siena by motorway and 4 lane highways.

Tuscany is much better seen by automobile. We especially liked visiting San Gimignano after all the tour buses had left, Volterra and Certaldo.

You can turn the car in at the Florence Airport or the Hertz location on Sansovino--2 miles from the train station. You won't get close to the ZTL downtown at either place.
The best way to get to Venice is by fast train.

Definitly pick the car up when you are leaving Orvieto and turn it in when you get to Sienna. IMHO you for a three day time span, you have way, way too much jammed in. T o do what you have planned would need a car but you will be spending too much time driving and no where near enough time seeing. Drop Assissi, Montepulciano and Cortona and you have three laces each worth a day and in a linear sequence. For that matter drop the car and go by train. If you do decide to rent a car, don't forget that you must have and IDP.

Great Planing !Your choose is good and affordable .

Personally agree with most of the others. The plan involves a LOT of travel. For me that would not be a great plan. We spent a week last year based about 30 minutes from Florence in the country side with a car. Big problem with a car is finding some place - convenient - to park it. Sometimes it took as long to find a parking space as it did to drive there. We had barely enough time to cover Tuscany without throwing in Venice and Rome. I would just focus on Tuscany with car and save Venice for the next trip. And August will be hot, humid, and crowded so you need to allow more time to see and do things. Since you are from Houston, perhaps heat and humidity is not that big an issue for you.

PS It has been over a year, so far no traffic tickets.