Good question, A_Z. It's sure flat enough. As far as I know, no land speed tests have ever been done there, but apparently a guy named Waldo Stakes is planning to test his Sonic Wind Land Speed Record Vehicle with the object of beating the Thrust SSC's current record. I imagine land speed testers tend to prefer the Bonneville Salt Flats 1) because it's in the United States and 2) because it's at a lower elevation, both factors making it much more accessible than Salar de Uyuni.