Prague to Sofia tour in October

That simply isn't a two-week trip. You'd be covering over 3100 km (over 1900 miles). ViaMichelin estimates the driving time (no stops, no traffic tie-ups, no navigational errors, no searching for parking, etc.) at over 40 hours.

Two days would be three nights, and that is not enough in cities like Vienna, Prague and Budapest. I'd say it's inadequate in Zagreb and Sofia as well. (Well, maybe not in Zagreb unless you want to track down all the art museums.)

The car will be burdensome at most of your destinations; you'd just be paying to park it.

You need to rethink this. It's several trips if you're limited to two weeks per trip. You can break the trip up geographically, covering fewer places and fewer miles per trip. Or you can separate the easy-to-reach major cities--visiting them by rail over as many trips as you need for them--from the challenging places that might benefit from being part of a driving itinerary. I think the second approach would yield the best overall results, and probably the least expensive, but I have no experience renting a car in Europe.

If you opt for a separate trip connecting a lot of the smaller places, do not expect to cover ground as fast as suggested by the ViaMichelin time and mileage I've cited above. The small places are likely to involve time on secondary highways, and you will not average 47 mph.

Edited to add: It's a real shame to go all the way to Bulgaria and just see Sofia. Plovdiv and Veliko Tarnovo, at least, should be included.

1) I’ve known people who have made long and fast driving trips and have been happy they did, so no criticism of the intent but I think you are pushing the possible with this one.

2) A fast but reasonable two week driving trip might be Czech Republic, Austria, Hungary, Croatia and Slovenia; but you are still going to miss a lot.

3) Another week for the Balkans and another week for Romania and Bulgaria; but again that would be rushed.

4) Tie them together with flights to major centers and start and finish at that point.

Thank you for the information.

I mapped this out as two trips: one a circular route from Prague, and the other a circular route from Sofia. If I don't delete any of the locations listed above, the total driving is not that much less. Actually about the same travel time when you figure car rental drop off, airport check in, flight time and car rental pick up at new location.

If all of these locations are worth visiting, maybe the best plan is to stretch it out to three weeks?

Or, do I get a really good Eastern European experience visiting Prague, Budapest, Bratislava, Sofia and maybe Belgrade?

1B) That seems to work better, and 3 weeks would make a world of difference.
2B) Its all preference, but if it were me, I would look at these three options as they are a pretty good cross section of Eastern Europe; and choose two for a 3 week trip; these are just route points, there is a lot in-between each; you have a lot of research to do.
3B) Option One: Prague to Vienna to Budapest to Graz to Salzburg and back to Prague (actually if you begin and end in Budapest you might have better discount flight options for the next leg)
4B) Option Two: Zagreb to Dubrovnik to Podgorica to Sarajevo to Zagreb (again, you might find it better to start in Podgorica or Sarajevo --- I am thinking Wizz Air)
5B) Option Three: Bucharest to Sofia to Plovdiv to Varna to Bucharest
6B) I know that’s only about half the countries you wanted to cover, but for me, I would get more out of it (I have been to every place I mentioned, so if you need more input, PM me).

Good that you include Novi Sad as part of the itinerary, see that fortress and the museum there, a most historical site indeed. There is public transport going from Belgrade to Novi Sad.

After doing more research, I have learned that I can't rent a car in Prague or Vienna and travel to Bosnia, Serbia or Montenegro.

So I'm going to scale this trip back and use public transport whenever possible.

Thanks to all for the great advice.