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.