Mine did not have an average MPG display. It is an early production base model (non-limited), non-nav highlander hybrid 4WD.

I bought a new version of scangauge (scangauge ll). Here's what I found.

Trip #1:
highlander hybrid = 28 MPG
wife's 2000 elantra = 27 MPG (exact same route)

Trip #2: HiHY = 33
Trip #3 = HiHY = 38
Trip #4 = HIHY = 35

Not bad for a beginer hybrid driver with less than 200 miles on the odo.

The good thing about the scangauge ll (aside from many features) is that at the current ave MPG setting, it resets everytime you turn the car on so it invites you to beat your last trip's mileage.

Mark