If a car moves down a hill at a constant velocity, the momentum will remain the same. Here's why: momentum is the product of the velocity and the mass of an object. In this case, since the mass of the car remains constant and the velocity remains constant, then the momentum will also remain constant. A simple way to determine the direction of momentum is to pay attention to the velocity. If the velocity increases, the momentum also increases; if the velocity decreases, the momentum also decreases. The mass will never change unless you are measuring objects using Einstein's theory of relativity. In Newtonian/Classical mechanics (which is what this question pertains to) the mass never changes.