Car Advice

Myth-Busting: Should you fill your tank with fuel in the morning?

Another one where your mate will tell you it’s better to fill your car with fuel in the morning when it’s cooler, because you’ll get more fuel into your tank…

THIS IS ONE OF THOSE myths that has an element of truth in it but one that’s been spread backwards. Let me explain.

‘They’ will often tell you to only fill your car with fuel in the morning when it’s cool as you’ll get more fuel for your buck. And there’s an element of truth in that, because when fuel, a fluid, is cold it is denser; when it heats up it’ll increase in volume but not density. But…

Fuel, you need to remember, is stored in giant tanks beneath a thick layer of concrete  which acts like an insulator (as does the ground around it) and, so, keeps the fuel at a constant temperature. Meaning, even if it’s a hot afternoon when you fill up your car, the fuel coming from the bowser will be the same temperature as if it was a cold day. So, whether you fill up in the morning or afternoon the fuel coming from the bowser will be, more or less, the same temperature.

Sure, the bowser itself might heat up, but that’s why there are big shade-providing roofs over the top of them, not just to keep the rain off you when you’re pumping fuel. That said, yes, on a hot day, just like your garden hose heats up in the sun, the first couple of litres of fuel being pumped will be warmer than those deep in the tank underground, but it won’t take long for the cooler fuel to flow through the nozzle and equalise that initial flow of warmer (expanded) fuel.

And don’t go and try and argue that if it’s a hot day when the tanker arrives to fill the tanks at the petrol station then the fuel pumped into the ground will be hot, etc etc. Because the tankers are insulated and designed to maintain the fuel at a set temperature. And then, once it’s sat in the ground it’ll cool down too.

But, what can have an effect is pumping fuel into a hot car. Meaning, if your car is on, say, a quarter of a tank and has been parked in the sun all day then the petrol in the tank, despite its insulation will have heated up and expanded, creating vapour. This vapour is generally captured to try and reduce the pressure in the fuel tank. And this pressure in the tank escaping is usually what you hear when you turn the cap on your tank (the hissing sound). Ideally, you shouldn’t hear the hiss as the vapour from the expanding fuel should be captured by the evaporative fuel recovery system (EFRS) which is usually just a brick of charcoal and a closed line that allows this vapour to be directed back to and burnt off by the engine.

On a particularly hot day this expansion of the fuel in your tank might prevent you from filling up your tank to the same level as on a cold day. But while I haven’t studied this myself, I’d expect the ‘fuel’ loss would be minimal.

What you shouldn’t do, in this situation, is to keep ‘topping off’ your fuel tank after you hear the first click on the bowser. What that does is overwhelm the EFRS and eventually lead to it failing, which means fuel vapours are able to escape into the atmosphere. And that’s very bad. You’ll usually see an engine warning light or be able to smell fuel near your car as evidence that something’s not quite right, you should have it checked out immediately.

With the heat wave that’s hit much of Australia recently a friend asked me if fuel could explode in a petrol tank in the heat. The answer is, no. See, without a spark to ignite the fuel, petrol won’t self-combust until the temperature reaches 400-degrees C.


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Ben Tate
Ben Tate
2 years ago

Isaac.

Good story. My background is Telecoms. Copper cable faults can be located by measuring resistance to the fault but resistance varies with temperature. Underground cables are always around 20 degrees celcius summer or winter. Aerial cable temps vary wildly and the fault locator must be adjusted to compensate.

A STORY SUGGESTION: “The Future of LPG Cars in Australia.” I guess that the backbone of LPG as a car fuel were Falcon taxis. Many taxis now seem to be Toyota Hybrids. Where does that leave the owner of an FG Falcon that burns only LPG? Is the VF with factory LPG dual fuel? Will servos continue to sell gas if taxi fleets turn to Hybrids?

Alan
Alan
2 years ago

Agreed – there may be a small effect between seasons (i.e. mid-winter in Western Qld vs mid-summer) where the underground temperature could vary – but that’s much different from morning vs afternoon, and most of us can’t wait 6 months to fill up 🙂 .

I’ve had several cars which would “leak” fuel if filled completely. One was an old Sigma, which would have a distinct petrol smell if the tank was filled completely and then parked in the sun. But more recently, my FIESTA diesel would dribble some on the ground – at the servo – if filled to capacity. I would only fill to first click after realising that.

My current car’s manual states:
●Stop filling the tank after the fuel nozzle automatically clicks off.
●Do not top off the fuel tank.

Rye an
Rye an
2 years ago

I prefer to fill in the morning if driving some distance on a hot day…..certainly not leaving it full and parked exposed to maximum heat.
Higher temperatures mean less air density and thus drag……windows up ac on, perfect!

Doug Mullett
Doug Mullett
2 years ago

“What you shouldn’t do, in this situation, is to keep ‘topping off’ your fuel tank after you hear the first click on the bowser.”
When I first filled my new Commodore, I stopped at the first click. “Wonderful consumption,”, I thought. “Terrible technique,” I realised, as I found my car was only half full.
Next time I filled, I noted the amount filled at the first click and then stood in disbelief as a further 25 L went into what was supposed to be a 75 L tank.
Now I know: when my gauge reads empty AND DTE is zero, there is 20 L in the tank; there is about 20-30 L between first click and full, depending on the individual pump; always fill just before driving off a reasonable distance (at least 10 km). The charcoal canister has NOT been overwhelmed in 13 years, 440 000 km and approximately 500 fills.

PracticalMotoring
2 years ago
Reply to  Doug Mullett

Thanks Doug, that’s good to know and all cars are different. I had one that on the second click would overflow. Cheers Isaac

Isaac Bober

Isaac Bober