How does the mass of a car affect its motion in regard to potential/kinetic energy?

The potential energy of an object is a function of its position and weight. The higher the object is, relative to a reference datum, the higher its potential energy. Potential energy is given as `mgh` , where m is the mass of object, g is acceleration due to gravity and h is the height. 

Similarly, kinetic energy of an object is a function of its mass and velocity and is given as `1/2 mv^2` , where v is the object's velocity. 

Since both the potential and kinetic energy are functions of the mass of the object, a heavier object will have lower velocity for the same kinetic energy. For example, if two cars have the same amount of kinetic energy, the heavier car will have lesser velocity. Similarly, for two cars with the same potential energy, the heavier car would have a lesser height (with respect to a given reference datum).

Hope this helps.