Just took a trip from Iowa to Phoenix and got mileage ranging from 28 to 35. Oddly enough, the highest mileage was through the mountains in SW Colorado. It seems I picked up more on the downslope than I lost on the upslope.
The Focus is badly lacking in HP for climbing those moutain passes however, I felt like one of those logging trucks trying to make it up with the engine downshifted and going 4,000 - 4,500 RPM to maintain a speed of only 45-50. One of the times I wish I had a manual instead of an automatic.
If I drove the mountains regularly, a Focus simply wouldn't do it for me, at least not an automatic one. But cruising across Nebraska on I-80 at 75, getting 31-32 MPG with occassional use of the A/C was quite nice in the Focus.