That's just in the US alone.
Aerodynamics are a major factor for automakers to get the best fuel economy from vehicles. Manufacturers work tirelessly to make sure that cars cut through the air efficiently by focusing on external components like the side mirrors and windshield. However, there is one manufacturer installed option that could be drastically hurting your fuel economy: roof racks. Roof racks are typically fitted to wagons and SUVs (or hipster Volkswagens) for carrying large items like bicycles.
A new study from the Lawrence Berkeley National Laboratory has found that roof racks were responsible for .8% of light-duty fuel consumption in the US. That may seem like a low percentage, but that number represents 100 million gallons of gasoline. Having roof racks on the car can hurt fuel economy by as much as 25% depending on the configuration. You may not individually notice any loses, but as a whole roof racks are responsible for a lot of wasted fuel. The authors of the study recommend that automakers work to make roof racks more aerodynamic and create racks that are easier to remove. Minimizing the number of unloaded roof racks on cars could save 1.2 billion gallons of gasoline over the next 26 years.
The study even suggests an extreme measure where the government would issue regulations on driving with empty roof racks. This may seem crazy, but even the study admits that this would be a last resort. Still, 1.2 billion gallons of fuel being saved would be nice. This number is equal to the amount that 86,717 cars would use in that 26 year period. All of that, just from two little racks that hardly ever get used! Listen up, automakers, and make your racks more efficient.