As a general rule, if you want to save money, fill your car tank at night when it is cold rather than at noon when it is hot. The price of gasoline is fixed per unit of volume (gallon-liter) and not per unit of solid mass (Kg).
Assuming a high temperature difference between day and night of 20 ° C and a volumetric thermal expansion coefficient of 0.00095 for gasoline and 0.00125 for good quality benzene, a gallon at night increases up to 1.025 gallon at noon, which represents a saving of about 2.5% on fuel prices ($ 2.55 / gallon in the United States or $ 5.6 / gallon in Europe).
Is this saving worth it? If yes, how much will your bill be reduced in a month or a year?