Filling the tank at the end of the day or at noon may save you a few minutes in the next morning when you are in hurry to reach office, but it will come out of your pocket. Petrol is a blend of hydrocarbons with the chain lengths from C7H16 through C11H24 and all of them vaporize at temperatures below the boiling point of water. Petrol expands during the heat of the day and becomes less dense. When it cools in your tank at night it would shrink to a lesser quantity, and in the morning it would give you a quick start. However, you will end up having paid more for it, even though the full number of liters were pumped in.