According to the Science Channel, a heavier car moves at a slower rate of speed. To increase the speed of a heavy vehicle requires more force to move the car faster, using more fuel. A recommended way to reduce the weight of a vehicle and increase the car's fuel efficiency is to reduce the amount of weight by substituting lightweight car components, such as replacing steel with carbon fiber.
According to the U.S. Department of Energy, reducing the weight of a car can reduce fuel consumption by an estimated 1 to 2 percent per 100 pounds. Removing excess cargo from the car is an easy way to increase speed by reducing weight without making modifications to the vehicle. The source notes that the percent of fuel consumption affected is directly related to the size and weight of the vehicle.
A smaller car will become faster and more fuel efficient by removing 100 pounds of excess weight than a larger, heavier vehicle. Discontinuing use of exterior cargo holders, such as cargo boxes, increases the aerodynamic properties of the car, increasing speed and efficiency, and can reduce fuel consumption by up to 17 percent, depending on the weight, location and size of the exterior cargo holder. An exterior cargo holder on the roof of a vehicle has more of a detrimental effect to the car's aerodynamic structure than one on the back of the vehicle that does not protrude over the roof.