Remove your roof racks and your top box when you’re not using it.
Holiday departures are often accompanied by “baggage extension kits” including on the roof of the car. The bicycle rack or roof racks and top case are a simple and convenient way to complete filling of the car. However, this solution is not ideal to optimize consumption.
First the air resistance is increased, and the aerodynamic is virtually canceled. Greater air resistance, beyond 90 kph greatly increases fuel consumption.
To this we must add the additional weight generated by filling the top box, up to an additional 200 kg in some cases. Overuse can easily increase 10% or 20% in extreme cases. Over a distance of 500 km, it is easily 15 € more. If you do not have another solution, do not hesitate to disassemble everything as soon as you arrive. Otherwise, priority should be given an extension of trunk solution, suspended on a hitch for bikes or extra trunk. Failing to avoid being overweight, you will preserve aerodynamics.