Roof racks are practically mandatory in outdoorsy regions – try finding an AWD vehicle in the Northeast without ‘em – but few pause to consider their environmental impact. It’s not negligible. Indeed, roof racks at their worst can give an economy hatchback the fuel economy of a crossover SUV. But there are ways to minimize efficiency losses, including a couple new roof racks designed expressly for this purpose. Read on for our lowdown on how to make your cargo-carrying as eco-friendly as possible.
Vehicle engineers these days spend countless hours making exteriors maximally slippery. Extensive computer modeling and wind-tunnel testing are employed to reduce the coefficient of drag
, which is inversely correlated with fuel economy. Not surprisingly, when you add stuff to your roof that the engineers didn’t plan for, the coefficient of drag goes up. Potentially way up.
A particular teardrop-shaped hybrid, for example, has a drag coefficient of 0.25, one of the lowest figures ever achieved by an automobile. But think about what happens when there’s a cargo box on your roof, or a pair of bikes, or even just roof racks by themselves. Heck, some folks have been known to strap a loaded dog carrier to the roof. Now the wind has to slip past multiple objects with different contours rather than one cohesive shape.
The net drag coefficient (yes, there’s a formula for that) naturally depends on your vehicle and what you’re carrying, but the effect on fuel economy is unequivocally negative. According to the EPA, a good rule of thumb is that a loaded roof rack reduces fuel economy by five percent
. According to message boards across the internet, that’s a rather conservative estimate. In one forum, two drivers reported reductions of 20-30 percent when using a roof rack and a cargo carrier. Another driver took a similar hit, dropping from the usual 48 mpg to an observed 37 mpg on a road trip.
But the good news is that such losses aren’t inevitable – at least not to that degree.
1. Remove Your Roof Racks When Not In Use
This message may be from Captain Obvious, but judging by the number of unladen roof racks we see on a daily basis, it’s not obvious to everyone. So here’s the long and short of it: if you care about conserving fuel and don’t use your racks every day, take the extra time and remove them after each use. Even without cargo, roof racks can noticeably compromise fuel economy.
2. Stash Cargo Inside or Behind the Vehicle When Possible
This is another bit of wisdom that may be hiding in plain sight. Bicycles, for instance, are designed to be disassembled quickly, particularly if you’re just removing the front wheel. Assuming you’ve got a vehicle with a tailgate, you could probably fit a disassembled bike or two in the cargo bay, as opposed to doubling the height of your ride by standing the bikes on the roof. And if you’ve got a trailer hitch, consider a platform bike carrier like this one
from Swagman. In the forum mentioned above, one driver claimed zero efficiency losses on the highway with a platform carrier, for roughly the same aerodynamic reasons that “drafting” behind a big rig improves fuel economy.
3. Make Sure Your Roof Racks Are Eco-Friendly
Roof-rack makers know as well as automakers that fuel economy is on everyone’s mind these days, and they’ve started to take action. Here are the latest efforts from two big names.
Featuring a trademarked textured surface called “WindDiffuser,” the AeroBlade
is Thule’s greenest line of racks yet, even if it isn’t exactly being marketed that way. Weight reduction due to the extensive use of lightweight aluminum will also help keep your MPG from plummeting. Notably, Thule claims that an AeroBlade is nonetheless one of the strongest racks you can buy, as it’s capable of withstanding over 800 lbs of force in a vertical load test.
Promoted as “the quietest, most aerodynamic and fuel-efficient line of roof racks in the world,” Yakima’s Wispbar
takes direct aim at conservation-minded drivers. A former Rolls-Royce aircraft engineer was hired to take Yakima’s racks to the next level, and we have him to thank for the Wispbar’s wind-cheating aerofoil wing profile. Like the AeroBlade, the Wispbar supposedly retains world-class strength despite its efficiency-first ethos.