Roof racks are handy additions to vehicles, giving additional space for bikes and luggage. But, they are also a real drag on fuel consumption and could increase your gasoline bill by as much as 25% according to new research published in “Energy Policy”.
In their study, researchers Alan Meier from Lawrence Berkeley National Laboratory (LBNL) and Yuche Chen from the National Renewable Energy Laboratory (NREL)*** examined the real world impacts of roof rack use in the United States. The racks are one of the most common and popular accessories on U.S. vehicles. But, their impacts are not included in fuel economy tests despite the fact that these racks are known to increase in fuel consumption because of the additional aerodynamic drag that they create.
According to Meier “I’ve always been intrigued by energy consumption that was somehow overlooked or ignored because, for example, it wasn’t in the test procedure…In this case the fuel consumption of vehicles with after-market accessories isn’t captured in the test procedure.”
In their study, Meier and Chen first compiled real-world usage data (compiled, through online forums and crowd-sourcing) and vehicle stock information. This work included watching national highway videos to estimate rack usage rates, vehicle stock, and vehicle miles travelled with roof racks (both empty and loaded). These researchers then used these data in a model to estimate the real-world impacts of roof racks.
All told, Meier and Chen found that roof racks were responsible for an estimated 0.8% of light-duty fuel consumption – or about 100 million gallons of gasoline - in 2015.
The authors also found that their results were most sensitive to the amount of time that racks were unloaded versus loaded. According to their research, the total miles travelled with unloaded roof racks is 4 to 8 times higher the miles travelled with loaded roof racks. In turn, while drag can be higher when the racks are loaded, the opportunities for unloaded racks could make the biggest difference in total fuel consumption.
Their results lead to some quite clear potential policy recommendations (with practical caveats. In their paper, Meier and Chen state that:
“For example, energy labels could guide consumers towards the most aerodynamic racks and encourage manufacturers to improve efficiencies of their products.
Additional policies could facilitate, or even mandate, removal of racks when they are not actually in use. Anticipated growth in rack usage makes such policies even more important.... Eliminating unloaded cross roof racks is a more effective strategy, compared with increasing the energy efficiency of roof racks, because vehicles with unloaded cross roof racks account for 4–8 times more VMT (hence, more fuel consumption) than vehicles with loaded racks. From a practical perspective, however, improving the aerodynamics of new racks may be simpler to accomplish than changing behavior.”
Meier and Chen's full study can be read here.
***According to LBNL "Chen conducted research as a graduate student at UC Davis and continued the work as a research engineer at NREL; Meier conducted some of this research while at the UC Davis Energy Efficiency Center."