For appx. 3 weeks I ran with my Quick-N-Easy roof rack on my vanagon,
having to carry some long stuff.   It's a full width two-bar rack, with the
cross bars made from Alaskan yellow cedar, appx. 2x4" and just a little
narrower, side to side, than my stock mirrors.  The 'bars' have large radius
corners.   I was working about 60mi round trip from home, but otherwise
driving in the same area and at the same speeds I do in 'real life' (when I
am doing what I want, not working for pay)

  My vanagon is a 5sp with a 2.0 liter Jetta inline gas motor and I've
always kept track of my fuel mileage, every tank for about 4 years now.
I've averaged ~23mpg really consistently.   If I use premium fuel without
corn, I get almost 2 mpg more.  If I drive at 75 all day, I get a few mpg
less.  On my 4 or 5 fill-ups while sporting that roof rack, my gas mileage
declined by almost 2 miles per gallon.

  Then, two tanks ago I took the rack off...it is Quick and Easy, so why
not, plus I'm out of work again so no need...The gas consumption again is
22.++ or 23 and 'small change'....  So there it is, a statistical sample of
one, but it is accurate.  I fill at the same pump, letting the same auto
shut off stop the flow (and spew some backwash out on the station's
driveway) each time.  I fill at about 275 miles run.  I had the same weight
load aboard.  I only carried a long ladder one time (30 miles) on the roof.

  In case anyone is interested in how aerodynamic drag sucks down the gas...