The average weight of the car will be less if you drive longer between
fill-ups. This will, as you suggest, lower your gas mileage, but I doubt
you can measure your fuel consumption accurately enough to catch this.
What I have found to be the biggest source of inaccuracies is that the
point at which the pump at the gas station shuts of varies wildly from
pump to pump. After noticing this, I always fill up at the same pump
every time (if at all possible). This gives me very consistent numbers.

ravelation wrote:
>
>
> escape2music@hotmail.com (Pete)
> wrote:
>
> >>Also, can you get a true reading of
> >>MPG when you fill up at half a tank?
>
> >No. How do you know you're at half a
> >tank? By looking at your fuel gauge?
> >There's no way you'll get an accurate
> >measurement that way. Fill up with gas.
> >Drive. Fill up with gas again. Then
> >divide miles driven by gallons purchased
> >(at second fillup of course).
>
> That's what he's been doing--dividing miles and gallons used. I guess
> the point I'm driving at is if you drive more miles on the tank, does it
> give a higher MPG than if you fill up often? IOW, will a longer useage
> period with more miles and gallons used create a better average?
> Is it possible the full tank of gas creates more drag at the first half
> of the fill up because of the added weight?