The heavier a car is, the more fuel it needs to cover the same distance. A roof rack is useful, but it increases the car's weight and – due to increased air resistance – significantly impacts fuel consumption. Depending on speed and load, the increase can be up to 30%. Even an empty roof rack increases consumption, albeit less significantly. Therefore, only use roof racks, ski racks, and similar devices when they are truly indispensable, and remove them when they are no longer needed.
For the same reasons, avoid using the trunk as a "storage" area and remove anything from the passenger compartment that is not needed on board. This also improves safety: in the event of an impact or sudden braking, unsecured objects can strike the driver or passengers with a force up to 50 times their actual weight.
