I would like to add the thuli roof rack, but I don't like that it is a permanent solution. How much does it affect the MPGs on the highway. If its only 5 or 6 mpgs that wouldn't be too bad, as long as it isnt affecting the city mileage when it is empty.
From personal experience, roof racks without a bike on cause a substantial impact on mpg. I usually assume 10% on the highway for a regular vehicle, I would suspect given the focus on aerodynamics in conjunction with the low power output on the PC - you would be looking at closer to 15% without anything on it. With a pair of tri bikes I would say 5mpg on my old 3 series wagon's. I wouldn't be surprised to hear 15mpg+ loss.
