Hand calculate, My MPG indicator on the dash sometimes looks right when going a steady speed, say 60 MPH, but I have seen readings of 35 MPH going down hill with no throttle, so I can brag about that but know better that it is just a instant calculation made by the computer for that time period and when I start back up that hill with a low gear and full throttle it will fall way back down. So the computer will deal with averages, hand calculate over several tank fulls and you will get pretty close to your actual MPG.
