As the other have said, there is absolutely no difference in MPG or performance going with a full, half or empty gas tank, except for the unmeasurable difference due to weight.
With my car, when the "low fuel" light comes on., I still have about 110km (70mi) autonomy (yes, I have tested it).
I personally prefer to stop at the gas station for a full fill-up than stopping for a 5-6 gals. top off when the tanks falls below the 3/4 mark.