ALMOST everyone has that one friend who swears by it: fill up your tank first thing in the morning, when the air is cool and the petrol is denser, and you will squeeze more fuel into your tank for the same money.
It is the kind of tip passed down through generations, delivered with the confidence of a man who has never once been asked for a source.
Is it true that you get more petrol for your money by filling up in the cool of the morning?
Verdict:
FALSE
Let us give that friend of yours his due, because he is not entirely wrong. The physics underneath the claim is genuinely sound.
Petrol, like all liquids, expands when it warms up and contracts when it cools down. Colder petrol is denser, meaning a litre of cold petrol contains slightly more actual fuel molecules, and therefore slightly more energy, than a litre of warm petrol.
Since we buy petrol by volume rather than by weight, in theory, a litre pumped when everything is cold gives you marginally more bang for your ringgit than the same litre pumped in the blazing afternoon heat.
So far, so good.
Unfortunately, this is where his theory drives straight off a cliff.
The problem is that the petrol you pump is not sitting outside sunbathing.
Petrol stations store their fuel in large tanks buried deep underground, where they are heavily insulated from whatever the weather is doing at the surface.
Whether it is a cool 6am or a sweltering 2pm, the temperature of the fuel in those underground tanks barely moves.
Non-profit consumer advocacy group, Consumer Reports, ran exactly this test using an underground fuel tank of the type found at a typical petrol station.
Over several days, while the air temperature at the surface swung by up to 7°C between morning and afternoon, the fuel in the underground tank stubbornly stayed at a steady 17°C.
The temperature of the petrol coming out of the nozzle barely changed at all between the morning and afternoon fill-ups.
Even in the best-case scenario where the fuel temperature did somehow change, the effect is tiny.
The same Consumer Reports investigation found that when petrol warms up by around 8°C, it expands in volume by just 1%.
On a full tank, that works out to a difference of a few sen, at most.
Here is the part that finishes off your friend's theory completely.
To chase that theoretical 1%, you would have to make a special early morning trip to the petrol station.
If the station is out of your way, the extra petrol your engine burns driving there and back, especially a cold engine, which is less efficient, would almost certainly cost you more than the handful of sen you saved. You would be spending 50 sen to save five.
But here is where the Malaysian context adds a genuinely interesting wrinkle, and where the BUT of this verdict comes in.
In Malaysia, the time of day you fill up really can affect what you pay, just not for any reason involving temperature or density.
Malaysian fuel prices are set weekly under the Automatic Pricing Mechanism, with new prices announced each week and taking effect from Thursday.
This means the price you pay genuinely does change from one week to the next, and if you know a price rise is coming, filling up the day before a scheduled increase can save you real money.
Similarly, if a price drop is announced, waiting a day to fill up gives you the lower price.
That is a far more reliable way to save at the pump than getting up at dawn to chase cold molecules.
For most Malaysians filling up on subsidised RON95, which under the BUDI95 scheme has been held at a fixed RM1.99 per litre regardless of the time of day, the whole density debate is academic anyway.
The price is the price whether you pump it at sunrise or midnight.
So the verdict stands.
Petrol is indeed denser when cold, and a litre of cold petrol does contain marginally more energy than a litre of warm petrol.
But the fuel in the underground tank is not meaningfully colder in the morning.
The difference is a rounding error even when it exists, and any saving would be wiped out by the petrol you burned getting there early.
Your penny-wise, pound-foolish friend can keep his theory. The rest of us can sleep in.
Sources:
2. https://auto.howstuffworks.
3. https://data.gov.my/data-
