For years we’ve heard that pumping gas when it’s cold helps you save a few euros. A small daily trick, almost a secret handed down between motorists. Yet, as often happens, physics has a slightly different story to tell. And Vincenzo Schettini, the professor and protagonist of “The Physics We Like”, explained it with that disarming clarity that manages to transform even the petrol station into an open-air laboratory.
Density, cold and the myth of the intelligent full
The reasoning, in theory, goes: when the temperature of a liquid drops, its density increases and its volume decreases. It happens to water, it happens to gasoline. And if the volume contracts, some people think that refueling the car at night is equivalent to buying more fuel at the same price. Especially if during the day it reaches 15°C and, at night, the air drops towards zero.
In the laboratory the calculation works: going from 15°C to 0°C, a 50 liter tank could “contract” up to one liter less. Translated: about two euros saved. An attractive idea, almost worth writing down in your diary. The problem is that this scenario remains a theoretical exercise. In reality we almost never see that game of subtractions and contractions.
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The truth in underground tanks
The fuel from the distributors does not live exposed to the crisp night air: it is stored in underground, insulated and stable tanks, precisely to avoid temperature changes. The temperature of the petrol down there does not follow either the sunrise or the sunset. It drops a little, sure, but nothing resembling that drastic jump imagined in textbook calculations.
And this is where the “sly trick” deflates: the real saving, that of the concrete world and not of perfect numbers, is just a few cents. No miraculous full, no night loot. Physics, once again, is less spectacular and much more honest than urban legends.
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