Well lets put it this way. Last fall I traveled from the mid west to the south east. The trip was mostly flat at first, and mileage was averaging about 25mph up to the point we starting getting to the hills/mountains. At that point, my mileage started increasing until I reached around 28mpg. The reverse trip was the same. Started with high mileage through hills and the mileage decreased after I reached the flats. I stayed about 1 or 2 miles per hour under the speed limit the whole trip and obviously the speed limit varied.

When traveling under constant cruise control, the best mileage seems to be from short steep climbs followed by long, gentle descending slopes. The other way around, hybrids don't help much.