Yes.

You're right -- because a fill is not always the same, measuring multiple tanks (with the pump and the odometer) by adding up the gallons and the miles, and then dividing, is the most accurate. It may be the only measurement that is always correct.

For me at least, I can't trust the NAV average at all. Perhaps this is because of my daily uphill 8 miles on the dirt road -- I'm driving slowly but getting low MPG. Perhaps for others, the NAV display is off in the other direction because they normally don't get low MGP while going slow. Usually slow driving is city driving which is high MPG. If I'm right then this explains why for most folks the NAV average is a little higher than true MPG.

If I'm right about the way the NAV average works, the NAV average will be the same as true MGP if you drive the same speed for most of the tank.

Mark