This was widely discussed in the first few years of Gen 6 production. Most people report that the onboard “Average MPG” display is about 5 to 7 percent optimistic, as given by the factory, when compared to hand-calculated results ( “actual miles” divided by “actual gallons”). A more careful analysis shows that the discrepancy is non-linear with respect to driving conditions … e.g. stop-and-go city driving versus highway cruising.
Fortunately, there is a “calibration factor” that is accessible to dealer technicians and can be set for your vehicle, from -10% to +10% in 1% steps. On my 2016 Outback 2.5, my dealer set this variable to -6%, which has improved the long-term accuracy of the on-board display to better than +/-2%. YMMV.
If you’re curious, I’ve posted the entire fuel history of my Outback on Fuelly (see link in my signature). This log includes both calculated and on-board MPG values for each fill-up.
