I was wondering if anyone has compared the fuel consumption penalty between towing a luggage trailer vs. having a roofbox (Thule or similar) on the roof rack?
I have done a bit of long distance with a Venter 6 foot trailer (actually too big for my needs) and I was surprised at how big a difference it makes (looks like 1.5l/100 Km at 125km/h). This got me thinking on how a roofbox would compare.

Has anyone done the comparison?