I haven't noticed this phenomenom in over !5,000 mi. The MPG average has been consistant throughout each tankful, with the same highs and lows depending on a given day's route. The higher avg. in the beginning of the tank on weekends is indicative that the weekday driving route consumes more gas (i.e., more hilly--you only recover 1/2 of MPG going down a hill as ascending one if it's the same elevation both sides; fast freeway driving, or a lot of stop and go, etc.). That would seem to be the pattern causing the difference.

Now it's true that a newer tankful will show more variance in the MPG average and that a "mature" tankful is harder to budge, but overall average remains consistant for me.