This is nuts! I just got back from a ski trip with the 550. It was about a 500 mi round trip with heavy mountain climbing, switch backs, etc. Our average elevation was probably about 5000 feet but we did plenty of climbing and topped out at about 9000 feet. The air temperature also varied immensely from about 50 degrees at lower elevation to 0 degrees at 9000 feet when we arrived at 10pm. On the trip to the resort the computer average MPG was 19! I filled up before returning and calculated the MPG to be 18.6. The computer wasn't far off! On the return trip, I couldn't believe it, but my computer average was 22.5 MPG and that includes about 45 minutes of city driving!
The thing was a complete joy driving in the mountains. It never hunted for gears on the hills, responded to the pedal very well, and the cornering bi-xenons at night made me feel so much more confident driving those curvy roads. There were several places with rocks in the road way that I was able to see in plenty of time. When descending long stretches, I simply set the cruise control and let the car handle the braking so I could concentrate on steering. Amazing vehicle!