We have a prius. It averages 46mpg on our normal driving trends, but 48mph on long stretches of 65mph trips with the roof rack, but empty.

1 road bike, with a round tubed roof rack, and wind faring. I also had a bug / wind screen bike cover. On a trip of 500 miles we dropped to an average of 35pmg.

Giant drop.

1 bike, 2 bikes, forward, backward, road or mtn, seems to be about the same 10mpg drop every time.

We have a prius. It averages 46mpg on our normal driving trends, but 48mpg on long stretches of 65mph trips with the roof rack, but empty.

Example test: 1 road bike, with a round tubed roof rack, and wind faring. I also had a bug / wind screen bike cover. On a trip of 500 miles we dropped to an average of 35pmg.

Giant drop.

1 bike, 2 bikes, forward, backward, road or mtn, seems to be about the same 10mpg drop every time.