Filling up with gas on a hot day would waste money: true or false?

Amid the heatwave, a rumor has resurfaced: it might be more economical to fill up your gas tank at night, when temperatures have cooled down a bit. This persistent and widespread belief is based on perfectly accurate basic rules of physics.

All liquids expand as temperature rises, and automotive gasoline is no exception to this physical rule. Thus, the average coefficient of volume expansion is around 0.0012 per degree Celsius, or an increase of 0.12% in volume per additional degree. This physical property has practical consequences: a difference of 20°C results in approximately 2.4% additional volume. A liter measured at 15°C occupies 1.024 liters at 35°C while containing exactly the same amount of energy.

To understand the consequences for the consumer, it is a technical matter of the difference between volume and mass. At the pump, for the same volume of gasoline served, it will contain less energy ( fuel mass) and will therefore take you less far on the road.

However, in practice, the design of modern gas stations significantly reduces the impact of thermal expansion. The station's tanks are buried at a depth of 1 to 3 meters, where the temperature remains remarkably stable. Even during the most intense heatwaves , this underground zone maintains a temperature between 5°C and 15°C, thus protecting the fuel from surface temperature fluctuations.

The persistent rumor is a very old story... As early as 2008, an American consumer association (Consumer Reports) questioned the veracity of the benefits of nighttime gas filling. It demonstrated that the underground tank remained stable at around 17°C, making the difference in volume almost zero once the first liters (heated in the pipe) had been used. In the end, the minimal difference in density only saved a few cents...