Easiest way to get car in Rome and then onto hwy

I would take the train to Orvieto and when I'm finished there if I still need a rental I'd rent there.

Have a Great Trip!

You won’t need your car in Orvieto, as traffic in the city is limited and there are ZTL’s. Most people end up parking in a pay lot and taking the funicular or elevator up into town. Therefore, I agree with the previous poster — take the train to Orvieto, visit the town for as long as desired, without the car, and pick up the car as you leave Orvieto..

I don't know for sure but earlier posts say termini. I am assuming you are going elsewhere after Orvietto and that's why you need the car. I'm in the same boat. I did see another post about some of the rental agencies been closed on Sunday so you might want to check that. I've also read to get a car with an actual trunk and not just to hatch back because of theft if you're going to be storing luggage in the car and to get an international license from AAA before you leave USA. Troll the forums here there is more information about this and earlier post something mentioned outlying areas in Rome to rent a car but I figured by the time I pay for the taxi or whatever to get to that outlying areas you might as well just be easier to rent it from termini because people have said that it's easy to get to the highway and I'm going to try to be brave. There are a few rental cars agencies in Orvieto to so you could take the train there and rent there. but they don't have Avis which is what I want . May have hertz

Sunday is a problematic day for car rental and return. Many places in the cities are closed and the airport is the only option. Check to see what is possible on a Sunday.

I agree with those above on taking the train to Orvieto and renting a car there but if you're set on renting in Rome we booked a car at an agency on the ring road from Autoeurope and took a taxi from our hotel near Piazza Navona to the rental agency. Easy access onto the Autostrada.
Buon viaggio!

Take the train to Orvieto.
Short ride, and no worries parking.
Enjoy Orvieto, it is awesome.
Jeff