I'd like to believe the display gauge is the more accurate measure (it makes my mileage look much better ), but if the current pattern continues I can't. On any one tank, the hand-calculated MPG might be off significantly, but over the course several tanks those errors should average out. If they don't, then one of the measurements has a systematic error. And I think a 5% discrepancy in gas pump measurements would eventually get caught, so any error is more likely to be in the NAH instrumentation.