We edited one post and deleted/edited three responses to it. All opinions are valid and welcome, and should not be turned into a personal dispute..

Jean said that the train was often faster than driving. I would say that on this trip, it's always faster. Driving from Venice to Rome takes 6 hours according to Google Maps, under ideal traffic conditions. The train takes, at most, less than 4 hours. According to ViaMichelin, the cost of the drive would be €92, which includes €41 in tolls, and the rest in fuel; it doesn't include parking or rental car costs. A high-speed train ticket costs €99 if you buy it at the last minute, or as little as €43 if you buy it at least 2 weeks in advance.

Driving from Rome to Naples takes almost 3 hours. The train takes a little over an hour. The cost of the drive would be €37 (plus parking and rental car fees) while you could get a train for €31 (€48 if bought at last minute).

Of course, if you have a car full of people, the cost per person would be lower, and driving might end up costing less per person than the train, but for speed and convenience, the train always wins. It takes you right into the center of each city. I live in Italy, and don't know any Italian who would make these trips by car unless there were some reason why they absolutely needed the car at destination.

If bought in advance €43 is high for Venice/Rome. I've routinely paid barely €20. My last trip was just under €30 in first class. That felt expensive.

Rome to Naples can be less than €10

I based my estimate on a day two weeks out, at a time of day I considered reasonable. I only looked at Frecciarossa trains, because they were the ones I had used for the speed comparison. And I ignored peripheral stations, such as Mestre and Tiburtina.

Just now I looked at a day in June, but I don't see any prices much lower than those I had already stated. Where do you find a €10 fare Rome to Naples? Even the regional trains cost more than that.

Those prices aren't unusual with Italo. Right now June has tickets starting at €12.90 without the discount code. €10.30 with the code that's about to expire in a couple of hours. This weeks discount wasn't even very good.

That's either very early or noonish. The only more expensive ones are mid morning.

I didn't check Italo, and should have.