We took our Prius over the mountains a few weeks back. I was looking forward to testing it at the extreme end of its design envelope, with a bulky cargo carrier to boot. This gave me an opportunity to see how much highway mileage would be affected by aerodynamic drag. Yes, yes, I should have stuck to the speed limit, but by not doing so I preemptively squashed a bitching point leveled by hybrid hatas -- Prius drivers sticking to the speed limit are always getting in the way.
We nailed 40 mpg on the nose for a 260-mile trip that was 95 percent highway driving. I was pleasantly surprised. Just look at that blob on top of the car. I used the cruise control religiously and pegged the speed 5 mph over the posted limit whenever traffic allowed, which was most of the time.