I used to tool around with a big Thule ski coffin on the roof of my Subaru and knew that it was costing me fuel mileage, but I assumed that when I took the box off, the roof rack itself was not a big deal. It turns out that it is; a new study by the Berkeley Lab has determined that roof racks are responsible for 0.8 percent of fuel consumption. That doesn't sound like much, but when you do the math it turns out to be 100 million gallons of gasoline every year.
The press release puts this into context:
Use of roof racks requires vehicles to expend more energy due to aerodynamic drag. While there have been studies of their impact on individual vehicles—depending on the configuration, the fuel consumption penalty can be 0 to 25 percent on passenger cars—this is the first study to estimate impacts at the national level. Moreover, use of roof racks is projected to increase given national travel trends.“A national perspective is still needed to justify policy actions,” the authors write. “For comparison, 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.”
In other words, just removing our roof racks would be a bigger deal than introducing hydrogen fuel cell cars at a cost of how many billions of dollars of infrastructure. The researchers think roof racks should be designed to be removed really easily, and that there should be rules:
The researchers estimated that a government policy to minimize unloaded roof racks (admittedly extreme) in combination with more energy-efficient designs would result in cumulative savings of the equivalent of 1.2 billion gallons of gasoline over the next 26 years.
I don't know why they think a policy is extreme; that's a lot of gas and a lot of carbon.