After spending a good amount of time driving through Patagonia, I think this little bit of advice would be good for anyone who is thinking of doing the same thing. Expect a difficult drive even if you stick to the paved roads, and keep an open mind as always.
The distances in Patagonia are great, and even when you finally reach a town after hundreds of kilometers of nothing, you might find that you are just in a small outpost with one gas station. Though you might have enough to get you to the next town, you should always try to top off, because you never know if the only gas station in town actually has gas that day. This was a problem for us in our trip, and we had to struggle to find gasoline to continue.
Wind is going to be a strong factor on the drive. As you head down the paved roads at a smooth 120 kph, you'll feel the car being knocked around like a toy, and you have to actually fight against the steering wheel to stay on the road. While I was driving, I generally always had the wheel at 10-11 o'clock or 1-2 o'clock, even though I was driving straight. You have to really pay attention because you will most likely get bored pretty quickly. The landscape is unchanging and radio stations are way out of reach. Bring CDs you can listen to over and again, and try to get some good conversation over the sound of wind against the car.
Other dangers on the road are animals. Though hardly anything lives out there, there is a large number of guanacos and sheep that roam around with freedom. From time to time these animals will get right on the road, and you have to give them right of way. The guanacos will usually clear out quickly, but farther down in the Santa Cruz Province they must be stupider because they actually walk into the road as you drive towards them. Then you also have rabbits, road runners, ostriches, and other species here and there.
When driving on the dirt roads, you really need to measure the quality of the road itself to know how fast to drive. Some are better than others, and I was able to get up to 60 kilometers an hour. Others were terrible, and we struggled at 30 kph. There are large stones all over the road, and you need to find the grooves in the middle to sail through, though those lines are also sometimes blurred. Rocks bump up to the bottom of the car and it feels like a bomb is going off, and throwing in the wind can push the car around even worse. Once in a while a big dust storm will come through and you have to stop the car and wait because there is zero visibility.
On the rare occasion that you pass a car, definitely slow down to a near crawl, and then there are two schools of thought. You can either get as far to the edge of the road or get as close to the other car as possible without hitting. The idea is that rocks will generally fly farther out, so if you get closer the will pass by. Either way, expect to have some dinks and cracks in the car by the end of the trip. And make sure you have a spare tire with you.
With every long road trip you want to be prepared. So get the gas filled up, bring water and snacks, and get your directions down. Luckily it's generally a straight shot in one direction with few roads to screw you up, but as you pass through towns the roads change names and you need to pay attention to get through on the same route.
So if you do decide to drive through Patagonia one day, you now have a bit of advice from someone who has done it. Get ready for a long and boring drive.