Now that we are back from our trip, I thought I would mention what we did in case anybody was interested in this day trip. I rented a car from Hertz at Vienna's Haupbahnhof or main train station. I brought my GPS from home because it had European maps and I am used to it. I chose Hertz because my credit card offered a discount and it was cheap. I had an International Driving Permit which made the rental process a little easier, I think.
I rented a Ford Focus wagon for € 59.39 when the Hertz office opened, drove to the hotel, and picked up the rest of our party.
Driving to Mauthausen was easy. The autobahn was safe and quick.
Mauthausen was truly incredible. The emotional impact of seeing where the suffering happened was unforgettable. I loaded the Mauthausen visitor app on my phone which provided a lot of information. After visiting the camp, we drove down the hill and turned up a small road to the bottom of the quarry. It was a short walk from the small parking area to the foot of the stairway of death.
We took more than four hours to see the memorial which is more than the average person would take. We read every display and took our time.
Walking was easy. We parked about 100 meters from the entrance.
After returning to Vienna, I dropped off my passengers at the hotel and drove back to the Haupbahnhof. I put € 26.80 worth of diesel in the tank before parking it. I took a lot of pictures just to be sure and dropped the keys in the box.
So for less than the price of one guided bus ticket, the three of us made the trip. The roads from the Haupbahnhof to the outside of town are easy to drive. Our hotel in the inner part of Vienna was not. I would recommend that everybody goes to pick up the car for that reason.
I hope somebody finds this information useful.