Imagine a bullet projectile penetrating through the atmosphere. Its external shape predicts its rate of velocity (speed) and distance of travel before falling to earth.
Aerodynamic drag is the resistance of the air to forward movement, called "air resistance." This is a factor of the shape of the vehicle (drag coefficient and frontal area), the objects which stick out (mirrors, bumpers, and antennas), the amount of turbulence at the rear of the vehicle, the nature of the vehicle's skin surface, and the amount of air going through the vehicle for cooling and ventilation. The less resistance, the faster the vehicle penetrates the atmosphere (air friction = velocity x velocity). The slower the vehicle penetrates the atmosphere the greater the amount of power needed to overcome this drag. A pickup truck is about as aerodynamic as a brick, with its blunt frontal area and restrictive tailgate-it seems it doesn't have a chance.
As a pickup's leading edge penetrates the atmosphere, the air passes on top and underneath, which according to the shape of these surfaces will determine the coefficient drag of the vehicle. With a blunt nose, the air is broadly separated as it travels over the hood and up the windshield A-pillar angle and over the roof. Directly behind the cab, a low-pressure area is developed as the velocity of the airflow begins to drop. Also, it begins to create a high-pressure area just after clearing the tailgate.
Installing a tonneau cover to the bed will reduce the coefficient drag of the pickup. As the airstream passes over the cab, it tends to drop due to a low-pressure area developed right behind the cab. With the addition of a tonneau cover over the bed, it will deflect the airstream as it continues over the bed. It has been proven that installing a tonneau cover will increase fuel mileage by 10 percent. Many people figure lowering the tailgate would relieve the high-pressure area in the bed, due to the obstruction of the tailgate.
But, to our surprise, the high-pressure area forms just after the tailgate. This discovery eliminates the theory of a tailgate hindering the truck's airflow. Actually, when the tailgate is lowered, it causes airflow buffering which causes drag.
In conclusion, to obtain optimum fuel mileage, there are many variables to take into consideration and each of these presented will help you reap the benefits of more miles per gallon. A well-maintained and tuned engine is the major contributor to achieving maximum fuel mileage. Go outside, pop your hood, and see what you can do to make the most out of your dollars.