Uspallata is a city in Argentina in Mendoza province. It's the last major town before the Paso Internacional Los Libertadores border crossing to Chile.
North towards Barreal and Calingasta in San Juan province
This road is brutal, but so worth it. Bring enough water, sun cream and food, especially in summer! 4 Liters is the recommended minimum.
You'll have to take the Ruta Nacional 149. Walk northward out of town, past the military base on your right hand side, until there's a large street to your left with a sign saying "Calingasta 150 km". Not that bad, you think. Go into this street and wait at/before the bridge in the pleasant shadow. There's very, very few people who take this road to get to San Juan province, and Barreal isn't exactly the center of the universe either. The shadow should last in this spot till very late in the afternoon.
If someone stops, they might want to take you to San Alberto, a hamlet 14 km away. There's one little shop (Almacen) there, that has no consistent service. Don't depend on it. You can walk northward till the bitter end of civilization, where you'll find a hotel named Hotel Pukarainca. The staff will refill your water bottles with drinkable tap water for free and try to convince you to stay there for the night, as what you're doing is hopeless. If you're here in the afternoon, there's a little tree on the west ("wrong") side of the road, where you can stay in the shadow all day. It's on the entrance to someone's ranch, but they seem to be never there. From 13:00 onward, the shadow will grow more and more towards the road. Mind of a Hitchhiker chose this more adventurous road to San Juan province in January 2017 (height of summer) and sat on her backpack till 19:00, when a truck in a convoy of trucks stopped to pick her up and bring her to Barreal. These were also the first and only trucks that used this road. She started in Uspallata at 9:00 in the morning. The traffic drives really fast here, so be alert when a car appears on the horizon and run to the road (with your sign) to make them aware of your presence and wish to be taken to San Juan.
Once you're in your ride, make sure not to be dropped off at the province borders (El Limite con San Juan). The Mendoza-San Juan province border 60 km from Uspallata is nothing more than a road sign. From Uspallata to the border, there's about 40 km of unpaved road (ripios), which is also why few people take this road. From the Limite onward, the road is paved and rather smooth. Once you're in Barreal, be dropped off at the camping municipal (around 30 pesos, January 2017), or like Mind of a Hitchhiker be dropped at the YPF gas station, get food, a bathroom break, fresh drinking water, and hitch through the night to Calingasta. Between Barreal and Calingasta is a hamlet called Sorocayense, which is where said hitchhiker freecamped in the Pampa for the night.
West towards Puente del Inca and the border with Chile
The Ruta Nacional 7 continues through Uspallata and makes a sharp left turn at the YPF gas station. There's many many trucks passing through here all the time. In summer, many vacationers make their last stop here before crossing.
South towards Mendoza
Everyone at the previously mentioned YPF gas station who is not going to Chile, is very likely going to Mendoza.
Accommodation and Sleep
The Camping Municipal is for free (January 2017) and one only needs to be registered, which is done after darkness by a loud individual with a clipboard. Mind of a Hitchhiker camped here with her drivers for the night, before separating.