Originally Posted by
Boudicca
Bike rack, specially a roof rack, adds drag and cuts fuel efficiency. If you aren't using it, don't leave it there.
That's not only true but it makes more difference than I would have suspected, at least on my Saturn SL1.
At speeds over 70 MPH with just the roof rack, no bike, I lost around 6 MPG. It would probably make less difference on a car that's not so under powered. Driving around town, which for us is almost all of the time, the mileage loss wasn't noticeable but mileage is pretty hard to figure very accurately in those conditions.