If a car is capable of generating more downforce than its weight, it's capable of driving upside down. But no one has actually attempted this, because with conventional aerodynamic devices—wings, diffusers, etc.—those sorts of downforce levels are only achieved at extremely high speeds. Downforce rises with the square of speed. So building some kind of tunnel to get a car up to necessary speed–and drive upside down—has just been a pipe dream.
