OP, what happens if you reset the onboard MPG calculation in the middle of smooth, not uphill, highway driving. That is excluding the highway entrance accelerating part and after the engine is warm.
When I do it, the refreshed MPG of my 28i X3 is usually above 28. I look at it and smile. And when I exit the highway and start driving local, I switch off the mpg calculator and pretend the number will stay.........