Well, let me just say my Prius usually gets 50 mph on the highway. The last time I did a road trip with a bike on the hitch rack, I filled up shortly after starting the return leg, and it got 35 mph coming home. I discount 5 mpg or so for the headwind, and come up with a 20% drop in mileage.

Since then the children moved away, so I can usually lay the bike in the back.