I found that I seem to get better mileage if I fill up my car between a half and quarter tank.
The more air in your gas tank the more potential for evaporation. Large gasoline storage tanks have a floating cover to prevent this. On the other hand, gasoline does weigh 6 lbs/gal so an extra 10 gallons adds 60 lbs of weight to your vehicle and decreasing mileage. I suspect that the weight would hurt mileage more than evaporation but you are probably splitting hairs either way. Since filling up your car itself uses gas (loosing your momentum braking to go to the gas station, extra gasoline to start your engine and recharge your battery, and then to accelerate back to road speed) I think the best strategy is to fill up then drive as long as practical on that tank.
it's a bad idea to run your car on empty. Most vehicles made whithin the past 25 years or so (educated guess) have the fuel pump located in the fuel tank (electric) as opposed to being mounted on the engine block (mechanical). When you run close to empty the electric fuel pump can starve for fuel at times, (when turning a corner for example) and eventually "burn up" your fuel pump because the fuel pumping through it keeps it cool. "starving" the pump can lead to an expensive repair.
I asked my mechanic and he says it doesn't make a differance, cuz I thought the same thing
I don't think it matters, its gonna use as much gas as its gonna use regardless
Well I have heard that if your tank is low you loss fuel to evaporation.