Great question — answering as the UphillTech team:
ConsumerReports.org has done the analysis . Obviously it'll be different for every car and shape and size of roof rack, and contents of the rack, but in their experiment: 2013 Honda Accord (4-cyl.) MPG (Miles Per Gallon) @ 65 mph = 105 km/h ---------------------------------------------- No rack 42 mpg = 5.6 l/100km Empty rack 37 mpg = 6.4 l/100km Empty rack and wind deflector 35 mpg = 6.7 l/100km Rack with two bikes and deflector 27 mpg = 8.7 l/100km So that gives you an indication that it can add close to 50% to your fuel bill, depending on the load.
Full answer by Mark Mayo: https://travel.stackexchange.com/a/26049 (CC BY-SA 3.0).
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