Gravity your kinetic energy when you are driving uphill and when you are driving downhill.

Gravity decreases your kinetic energy when you are driving uphill since the direction of motion is opposite for both. Driving uphill is force going upward while gravity pulls object down. When it is going downhill, the car tends to go faster since the gravity helps the object to go down by adding another value to the total acceleration of the motion of the object. Using the forces of balance, an object going up tends to become heavier while object going down tends to become lighter because of the gravity factor. Another analogy is the motion of elevators going up and down that incurs effects to your weiight.