I own a HCHII which I purchased three months ago.
I routinely drive 75--80
mph on they highways here in NC, which are typically a bit hilly, and certainly not totally flat. I usually get 40-45 mpg even at those speeds, using the "pump and glide" method (I don't use cruise control because of the uneven terrain).
So, I can't complain about my gas mileage, even though I know it would be better if I kept my speeds below 70.
Aneil