For the extreme example of the full loop you describe where the car is upside down at the top of the loop, the car needs to be going fast enough to counteract gravity at the top of the loop. The formula that describes this minimum speed is V^2 / R = g (V is minimum velocity at the top of the loop, R is radius of the loop, g is the gravitational acceleration that must be overcome). Notice that when the car is vertical gravity is pulling the car backwards, which causes it to slow down. It will speed up again by the same amount (if you ignore friction) at the other side of the loop as it's vertical facing downwards. This situation is completely different to the deformed rubber sheet analogy, I'm not even sure why you brought it up.