Are roof boxes bad for fuel economy?
Thanks to the changes in aerodynamics that roof boxes cause, you will see a slight increase in fuel usage if you use a roof box. As well as the extra weight of both the box itself and the items inside it, you'll also be creating extra drag – also known as air resistance – which will force your engine to work harder to maintain speed.
However, that increase will be minimal. Thule, one of the biggest manufacturers on the market, estimate that carrying a roof box only leads to a 5% increase in fuel consumption, so you won't be stopping more regularly to fill up.
Roof boxes are designed to be efficient when it comes to air resistance, with tapered fronts that cut through the oncoming air and lightweight builds that are insignificant compared to the weight of the car and additional passengers. The only way a roof box could affect fuel consumption more significantly would be if it was packed full of extremely heavy items, so if you're only transporting a few bags or some camping equipment, you really don't need to worry about the effect on your car's fuel consumption levels.
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