Roof boxes are designed to be as streamlined as possible and Thule designs some of the most aerodynamic boxes on the market. They are a great option for vehicles that can carry something on the roof.
Towbar cargo carriers, on the other hand, have the advantage of being at the back of the car in a place that barely affects wind resistance at all.
Towbar trailer cargo carriers like the Thule Arcos generate little to no wind drag. This means that, with a Thule Arcos towbar cargo carrier, you’ll be using the same amount of gas or electricity as a vehicle without anything on it.
They’re a great option for those who do not want to carry anything on their roof.