Ok this is what I heard but can't confirm but makes sense to me:

1. Pump early in the morning because the gas is the coldest at that time. In the evening heat is still retained in the concrete thus pumping fuel late evening is still warmer then morning. In science molecules contract when its colder. Following that priciple you should get more bang for your buck when you pump in the morning.

2. The petrol tanks are also refilled in the evening and will stir up the petrol and moving the sediments, If you are pumping at that or after that time the sediments might be pumped into your car.

3. Pumping at lower speeds might take longer but there will be less evaporation. Its like washing your car when you on more water, more spray will be created especially when you hit the car.

4. Pumping half tank is better because there will be a point that the fuel can't evaporate due to saturation in the. Larger empty portion = more evaporated gas. Also its no fun when your tank shows empty and you are not near a station. Heh.

5. Safer to also pump in the morning as more people and robbers are less like to rob then.