I was sitting in the back of a pickup truck while tossing a coin up in the air. At first when I tossed it relatively low, the coin seemed to fall as if I was sitting inside of the truck. It fell straight down in my perspective because it had the same horizontal velocity of the car. To someone on the sidewalk, however, it would look as if it was moving in a parabolic motion. When I threw the coin higher, however, the coin encountered a wind force that slowed down the horizontal motion of the coin. This resulted in the coin landing on the road in back of the tailgate. This drawing illustrates the air flow.

When the tailgate is up, an air pocket is formed. This helps the truck to be more aerodynamic. If the tailgate is down, the air current will have higher pressure above the bed, increasing the drag of the truck in motion.