 My 6cyl 3 series averaged 28.4 mpg on the highway when traveling in the 80-85 mph range. The X3 is in the area of 18mpg.Now, if I slow down to 70 the mileage jumps up to 26+.Of course on the highways of AZ and CO where the limit is 75, traveling under the limit is somewhat risky as the normal flow is near 90. Overall I really like the X3, and I suppose 18 mpg on the highway for an all wheel drive isn't horrible, although my previous Explorer was over 20 mpg.Driving within normal limits I average 27.6 combined. I notice no difference in performance or MPG with different gas grades. So it seems as if the cheap stuff is just fine in the cool weather and mid-grade when it is warm out. 