Go on any highway and you'll see many vehicles with roof racks. Some are actually carrying things (bikes, kayaks, etc), but the vast majority are empty, and a good subset of those will never actually carry anything.
We know that aerodynamics matters a lot at highway speeds (the more 'slippery' your vehicle is through the air, the less your engine has to work to keep the vehicle moving), but what kind of impact does a roof rack have on MPG? That's the question that Darin at MetroMPG asked himself, and he conducted an experiment to find out.
The Roof Rack MPG Experiment
He compared the gas mileage of his car at 54.7 mph (88 kph) with 1) no roof racks, 2) an empty roof rack, and 3) a mountain bike on the roof rack.
He found that compared to the baseline of no roof rack, the empty rack reduced his MPG by -12.7% and the roof rack with a bicycle reduced it by a whooping -27.3%. And since air resistance increases exponentially with speed, the difference would have been even bigger at 70 mph (many people drive at that speed).
While the methodology isn't 100% scientific, and every vehicle will be affected differently by a roof rack, it gives a good idea of how big a difference a roof rack (empty and full) can make.
So please, if you're not using it, take the roof rack off your car (and forward this tip to your family & friends)! It's such a simple trick, like inflating your tires properly, that could save millions of gallons of gas.