I always thought that it was just my car doing extremely high mileage for the first 350KM (218mi) after i it fill up full tank with 37.5L(9.9gl). But i see some people are experiencing this too

I always get 4.2L/100KM(56MPG) for the first 350KM, and then after that the mileage value deteriorates to 4.9L/100KM(48MPG) by the time it reaches 800KM or 500mi when there's only one bar of fuel left. There's no change in my driving habits. I drive 95% city

My driving habits are conservative: meaning i dont really speed a lot unless i'm late for work, and i always leave the climate control with A/C on at 23.5C.

Actual hand calculated mileage at every fill is (37.5L at 800KM is 21.33Km/L or 50MPG).

This is a fairly easy hypothsis to test. Begin recoding a daily log of your MPG. After a couple of tanks, you'll be able to document the trend.

I top off my tank twice a week, Fridays and Sundays. This gives a way to see if there is a pattern in the partial tanks suggesting some MPG anomalie. The data suggests that if it exists, the period is about 200 miles or less. But the real answer is using a daily log to see if the problem(s) can be correlated to some specific and previously undocumented effect.

http://hiwaay.net/~bzwilson/prius/pri_tank.html

Bob Wilson

Operation Iraqi Oil Freedom:

Automatic, stock, project car.

My other 1500 cc car:

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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.