Updating this 11 year-old thread...
Consumer Reports asked:
What's the Impact of Carrying Stuff on the Roof?
They tested with a 2013 Honda Accord at 65 mph / 105 km/h.
- 42 mpg US - baseline with nothing on the roof
- 37 mpg - empty bike rack
- 35 mpg - empty bike rack with wind deflector (made it quieter, but increased drag)
- 27 mpg - two bikes on the rack (didn't specify deflector or not)
They also tested a 2008 Camry with a large car-top carrier...
- fuel economy dropped by 5 mpg
Source:
https://www.consumerreports.org/fuel...l-economy-now/
Moral of the story: carry stuff inside the car or on the back whenever possible!
