I am wondering if hybrids efficiency decreases when in the Rocky Mountains?

It seems to me that mileage might decrease when climbing to 11,000+ feet passes, like Vail Pass, not because of the altitude, but because of the steep ascent.

Conversely, the mileage would improve dramatically when coming down from such heights.

So, do the two balance each other out and still average a better than gasoline MPG?

Thanks for your thoughts.