I actually have a neat comparison study in play for this situation. My buddy Leif and I both have new Ford F250s with diesels. Same engine, transmission, similar shells and the same tires. I have a shell with Thule racks, and he has the most ridiculous exoskeleton rack known to mankind wrapped around a shell.
With 2 boats on top from Portland to the Gorge and back, I run about 18 mpg, he runs about 16 mpg. That's 5.55 g / 100 miles for me and 6.25 g / 100 miles for him, which is a 12.5% difference in fuel consumption.
Pictures of the two setups: