Laurentiu - that's a hilarious image. That's a biodiesel series electric limosine, right?
swartz77 - welcome to the forum.
One of our more data-conscious members has in fact documented the (very small) mileage benefit of reduced weight from a partial tank of fuel vs full, so there's some merit in only filling half way.
BUT - you can't change what you can't measure! If you're only estimating MPG, it seems kind of ... imperfect. Unless of course you have instrumentation (original equipment or ScanGauge) to get at least trip-level fuel consumption info.
If you want both the weight savings
and consumption accuracy, you could always fill to the top, then syphon half of it back into gas cans (adding it back as you use up what's in the car).