Venice-Florence-Cortona-Rome - How many days in each?

I don't see any need for 3 nights in Cortona so would either add a day in Florence or Venice instead. That being said, Florence is not the most car friendly city in Italy with it's well know ZTL's and the risk of zipping through one either at or to or from your hotel. I would suggest you look at Siena as your base for day trips as there is a lot more going on there then in Cortona.
A couple of trips ago, we did use Cortona as a 2 night base to visit Azzero and Greve[the Veranzano winery(loved it!) ]and then moved to San G as a base(2 nights) to tour there and Siena. Finally, I usually drop my rental car at FCO and take the train into Rome rather than loosing the time leaving it at Orvieto as many suggest.

Unless you're planning on doing some day touring from Cortona, I'd also suggest dropping one night there and adding to Florence. If you're interested, that will allow day trips to Siena, Lucca or other locations in the area. I would not add a night to Venice.

As the previous reply mentioned, you'll have to be extremely careful not to drive through any of the ZTL (limited traffic) areas in Florence, as a hefty fine will result from each violation. Renting from the airport would solve that problem, although airport rentals are usually more expensive than "city" rentals. Hopefully Roberto will spot this thread, as he knows where all the rental agencies are located relative to the ZTL areas.

I think your allocation is great, except for maybe, and this is only my own preference for smaller places over large cities, I would borrow one night (or even 2 nights if it were me) from Rome and give at least 4, and maybe 5 nights to Cortona.

It's not that Cortona itself needs 5 nights (less than a day is enough for Cortona), but with a rental car, from Cortona as a base, you can visit a lot of places within one hour or so drive. To name some of the closest to Cortona (in alphabetical order):
Anghiari, Arezzo, Assisi, Bagno Vignoni, Buonconvento, Castiglion del Lago, Castiglion Fiorentino, Cetona, Chiusi, Chianciano Terme, Civitella, Cortona, Gubbio, Loro Ciuffenna, Lucignano, Montepulciano, Montalcino, Perugia, Pienza, Poppi, San Quirico d'Orcia, Sarteano, Siena, Spello.

They are all beautiful and quaint, but of course you don't have weeks to visit all of the above, so I bold faced the most famous.

You can rent a car using a consolidator such www.autoeurope.com or www.kemwel.com (they are actually the same company after a merger years ago). Most of us in this forum use the above. They work primarily with Avis, Budget, Europcar, Hertz, which are the major companies in Italy.

You would pick up the car in Florence at the end of your visit just before heading out of town (any pick up location will do, but picking up at the airport carries additional charges). There is no risk of getting a fine in the ZTL on your way out. That risk exists only on re-entry, if you return the car to Florence (which is not your plan anyway).

You can return the car in Rome or, if you happen to choose Hertz from the above consolidator, you could even visit Orvieto on the way to Rome and drop off the car at Orvieto (only Hertz has a facility there).
Rome has many locations throughout the city. Which one to choose will depend on which rental company you select. We can help you there once you make that selection.

Cortona has a ZTL where traffic is restricted, therefore with a car choose lodging outside the ancient city center or at least that is accessible by cars (preferably with parking on the premises). There are plenty of countryside hotels and villas all around Cortona.

That is really helpful information.. and I think Roberto you do understand and share our preferences for countryside over city. We are in a little more unusual position in that we are choosing lodging we can get using timeshare points... through a rent a villa program associated with that.. so our choices are more limited, although still really good. I think Cortona is our only small town choice near Florence. We don't intend to drive or keep our car in any of the major big cities. I'll figure out car rental with your guide lines. I like the list of cities we can visit from your list.. Assisi I know is a must for us based on friends who have been. I think we'll consider adding a day to the Tuscany area.. but we can't go shorter than 2 nights anywhere because the lodgings we are looking at have 3 night minimums... so can't ramble around from town to town. I believe we could cut Rome by a day though.. because we will have enough City and Museum type places in other areas. I was thinking if we added a day in Florence we'd use it to take a train-based day trip outside of it.. I'll consider the options some more.. but this has given me some good perspectives to think about. If anyone else has favorite strategies to mention, I love having input.

I posted this earlier on your other thread ...

Some thoughts on your time in Tuscany after Florence, if you have decided on Cortona.
Day 1 - Drive via Siena (or Chianti) to Cortona
Day 2 - Excursion to Montalcino + Montepulciano
Day 3 - Excursion to Assisi + Gubbio
Day 4 - Pinch a night from Rome for an overnight in Orvieto (returning your car here)
Day 5 - Rome by train

One point to mention regarding parking in Cortona. If you want to stay right in town, that's certainly possible. For example, Hotel San Luca has a few parking spots in front of the hotel, but I'm not sure how many are available for guests or whether there's a charge for these? There's a larger car park just below Piazza Garibaldi that's accessible via two Escalators, and I believe that one is free. On the lower left of the photo, the stairs that are shown next to the arrow are the ones that lead to the Escalator and car park, and Hotel San Luca is on the upper right by the green astro turf.

I agree with Roberto, as usual. I loved Cortona and wished I had stayed there longer! With a car, that you can pick up outside Firenze and return in Orvieto (I only use Hertz as we had terrible ordeal with Avis) and travel the beautiful countryside. We drove to Assisi and a few small towns. Southern Tuscany/Umbria is a beautiful region. We stayed at San Michele in Cortona and since we paid in advance, got a great rate from the hotel's website. I would return there just for the room and its fabulous fresh breakfast! They also have a sister hotel in the valley with a pool and provide transportation there. Have a fabulous time!
Marie