Check out the Consumer Reports video below to see the consequences of going 55, 65 or 75 miles per hour. The CR team used a fuel meter to test out a few models on the highway. One of these test models was a four-cylinder Honda Accord LX. At 55 mph, they were able to get an amazing 50 mpg (and it's not a hybrid). At 65 mpg, it dropped to 42 mpg and at 75 mph, it went down to 36 mpg. The Toyota RAV4 and three versions of the Ford Fusion (including the infamous hybrid) showed similar results.
Loading up your car also hurts fuel efficiency. A bike rack drops the Honda Accord to 37 mpg; adding a wind deflector drops it to 35 mpg and propping up two bikes on the rack drops it to 27 mpg.
It turns out fuel saving options are more limited than we'd assumed. The technology in the powertrain matters, but if you really want to save fuel, you can lighten your foot on the pedal – or just take your bike off the roof rack and ride it to your destination.
<iframe width="628" height="360" src="http://www.youtube.com/embed/2uAjX5hm9cI?feature=player_detailpage" frameborder="0" allowfullscreen></iframe>