My mpg is all about how I drive. I tried Eco mode and paying attention to my right foot for about 20 miles on the highway and saw 27.5 mpg and it was still climbing slowly on my 35i. I just can't drive in this gas conserving mode very long.
Combined city and highway and staying in the sport mode like I normally do, I see about 21 mpg combined. Unfortunately to survive in a world of aggressive drivers, I find being in the sport mode my comfort zone. I'm still very happy with the mileage so far. I'm at 2800 miles and just pushed the car to the 6k rpm point after 2500 miles.
1979 Porsche Tobacco Metallic 911SC
1996 Porsche Guards Red 993 C4S
2003 BMW Ti Silver M5
2013 BMW X3 35i Carbon Black M-Sport