I have been researching a lot about petrol lately, and my findings have received acknowledgement by various stakeholders in the petrol business and corroboration from petrol station attendants. One of the reasons that made me embark on this venture was when it caught my attention that temperature difference is a big gamble for the petrol business, and since most people leave the house straight for the office in the morning and fuel later, fuelling stations end up making more while you end up getting less from your money. Really?

In the morning, temperatures are significantly lower than during any other time of the day, most remarkably in the early afternoon. That means that in the morning the petrol is denser than in the afternoon due to the colder temperature of the ground. In physics, when any liquid undergoes a temperature change, its volume changes proportionately to that temperature change. This change is governed by a given constant, and this is referred to as the coefficient of volume expansion. While external temperature changes may not affect other liquids much, petrol, being very volatile, expands much in the afternoon when the temperatures are warmer and therefore when you fuel at that time, the fuel will contract at a cooler time and will decrease in volume, so you will have enjoyed less value for money. One thing to note, however, is that any adverse temperature changes are as a result of the petrol sitting in the fuel dispenser, which gets warmed by the ambient temperature, but not that the fuel had warmed up and changed in volume while still in the underground storage tanks, as temperature variations below the surface are less pronounced than those on the ground. According to Craig Eerkes, former chairman of the Petroleum Marketers Association of America, the changes that occur at different times of the day are so minute to be almost nonexistent. This is because with modern technological advancements, just as fuel vapors in the car's fuel tanks which were formerly lost to evaporation are now being circulated back into the system to give a car more fuel economy, some modern storage tanks have double walls with which to keep the fuel at the same temperatures, with any changes being negligible. "If fuel is warm when it's delivered to a station, it'll be still warm when sold a few hours later." – Judy Dugan, Research Director for Consumer Watchdog. Also to note is that whether the fuel is denser or not, the power it can deliver per given unit will still be the same, and so in terms of power persé, the time you fuel does not matter at all.

We have seen that the reason why fuel may expand is due to sitting idly in the fuel dispenser rather than because the ground is warmer, and so the best move is to choose a filling station whose fuel hasn't been sitting idly for long. It is then that I still advocate for a morning fill-up, because you are not at liberty to know which pump has idle fuel and which hasn't, and so a morning fill-up is a sure bet since even if the fuel has been idle in the dispenser, it hasn't expanded in response to a heated environment.

You can fill up at six in the morning, at twelve noon, at six in the evening or even at midnight. It doesn't really matter, for as much as a 15 ° F change in temperature will cause only a 1% change in volume. But still then you may have other considerations. What's important is that you have the right information.