BERKELEY, California — Automobile roof racks can reduce fuel economy by as much as 25 percent, according to a new study from Berkeley Lab.
The study, Fuel Consumption Impacts of Auto Roof Racks, by Berkeley Lab researcher Alan Meier and Yuche Chen of the National Renewable Energy Laboratory, for the first time measured the effect of attaching cargo racks to the top of light vehicles.
The result: In 2015, roof racks were responsible for 0.8 percent of light-vehicle fuel consumption nationwide. That comes to about 100 million gallons of gasoline.
The study, published in the journal Energy Policy, was conducted using online forums and crowdsourcing to gather data. Then Meier and Chen built an energy-inventory model that took into account rack usage rates, vehicles miles traveled and the amount of fuel used, both with the rack loaded and unloaded.
Interestingly, they found that while aerodynamic drag increased fuel consumption significantly with a loaded rack, there was still a considerable difference even with an empty rack, indicating that owners would benefit from removing racks when not in use.
The authors suggest that racks should be designed to be more aerodynamic to lessen their impact on fuel consumption and that they could also be made easier to install and remove.
The study calculated that "the additional fuel consumption caused by roof racks is about six times larger than anticipated fuel savings from fuel cell vehicles and 40 percent of anticipated fuel savings from battery electric vehicles in 2040."
And since the researchers also determined that roof-rack usage is expected to increase by as much as 200 percent by 2040, due primarily to growing nationwide travel trends, measures to make them more aerodynamic and more convenient to use would result in significant cost savings to consumers.
Edmunds says: While roof racks provide convenient storage on car trips, this study clearly shows that owners can save fuel by leaving them home when not needed.