Some people might call it an urban legend, while some believe it to be true – if you fuel up your car at night or early morning, you get more fuel for what you pay for. This belief is backed by the principle that fuel expands when heated. So the theory is that if you fill up your car at night or in the wee hours of the morning when the temperature is cooler, you’d get more fuel for the same amount of money that you pay for. However, while it does makes sense, but the question is, how much could you actually save on refuelling at night or early morning?
In theory, it's true that fuel does expand when heated, making the liquid less dense. So should you fuel your car in the morning, the fuel is dense and you would get more amount of fuel. However, there is one particular flaw in the theory. Most fuel stations or pumps around the world store petrol or diesel underground in large tanks beneath a thick layer on concrete which acts like an insulator and keeps the fuel at a constant temperature. Therefore, the rising temperature of the day plays no part in the density of the fuel and the fuel from the bowser will remain the same as it would on a cold day.
One might argue that the bowser might be hot and the fuel in the pipes would also be less dense. But similar to a garden hose, the first few litres of fuel might be warmer than the fuel coming from deep within the tanks, and it wouldn’t take long for cooler fuel to being pumped out from the bowser. On a busy day, when the station is regularly pumping fuel into vehicles repeatedly, the fuel coming through the pipes and the bowser will not be affected by the heat at all.
While it is true that fuel expands when heated, the change it would make in terms of cost saving is actually quite marginal. If you’re someone who needs to go out of your regular way to refuel especially at odd times of the day, you’d probably end up spending more money on fuel than you would save in just going the extra miles.