I frequently drive up Hwy 88 in CA from the Stockton area to Jackson in the gold country. I find that climbing the grade (which is long, steady climb of about 40 miles) the Prius gets better mileage up the grade then down. But I also find that to be the case when the wind is blowing eastward toward the Sierras. On one occasion with no wind I got a fabulous 60 mpg going downhill all the way to the Oakland area, including going up over the Altamont and Dublin passes. It seems for me the wind makes the most difference in mileage, and not the climbs I am making.