Improve your gas mileage by 10% instantly, without modifying your car or driving habits! If I saw that in the subject line of an email, it’d get deposited, unread, in my junk folder along with the impotence-related drug ads and mortgage plugs that take up a sizable fraction of my inbox. It just so happens, however, that the claim above is true, and the result of one of the easiest things you can do to reduce emissions on a day-to-day basis: remove your roof rack when you’re not using it, and use it only when really needed.
On the roads around the bay area, you’ll see hundreds of opportunities for savings — mostly empty cars with a loaded roof box, or just empty bike and ski racks left on as people commute. According to most sources, a roof rack will reduce your fuel economy by 5%, and, in the examples above, that reduction is totally unnecessary.
The roof rack may be a status symbol among the young and active, but anything protruding above the roof of your car is an aerodynamic disaster, and, as anyone who has ridden a bicycle into a headwind knows, fighting wind resistance consumes huge amounts of energy. The increase in fuel consumption, and corresponding emissions, isn’t linear either: the graph is exponential, curving steeply up as speed increases, meaning that the government’s 5% estimate may be quite conservative at modern highway speeds.
While an unloaded roof rack may not be quite as bad, a rack with bicycles or boats is much worse. The gas mileage in my Audi drops over 10% (27mpg to 24mpg) with two bikes and a box on the roof, and other anecdotal reports support this larger number. This means that for a weekend mountain biking trip to Tahoe I’d be burning almost two and a half gallons of additional gas and producing about 50 pounds of extra carbon compared to going rackless.
The solution to this is quite simple: only use the rack when space is at a premium. I can pack two mountain bikes, two people, and all our equipment for the weekend into the back of my Audi, and it’s a small sedan. Packing inside the car also greatly reduces the risk of theft or damage from weather and flying debris, and, depending on your insurance coverage, may be the only way to have your bicycles covered under your auto policy.
The flipside of the rack argument is the potential for racks to reduce the total number of cars on the road. If you can get four buddies and four bikes into and onto your car, instead of taking two separate vehicles, the increase in emissions is offset by the great decrease in per passenger fuel consumption, not to mention traffic.
So carpool if you can, but, unless you’ve got skis sticking out the window and bike wheels duct-taped to the bumper, skip the rack and save the gas. De-racking may take an extra couple of minutes, but represents a small effort for emissions reductions that add up every day you’re in your car. Next time you’re commuting, watch for empty racks and do the math: together, with no sacrifice on a day-to-day basis, we could make a surprising dent in the pollution we all produce.