Popular tips on filling cars with petrol don’t offer big savings

21 June 2022
What was claimed

Buying petrol first thing in the morning provides better value for money as cold fuel has a higher density and therefore greater volume.

Our verdict

False. Petrol stations store their fuel in underground tanks where the temperature remains constant, even when the outside weather is particularly hot or cold.

What was claimed

Filling your petrol tank more slowly reduces the amount of fuel lost as vapour or through evaporation.

Our verdict

While at least one study does support this claim, the amount lost as vapour when refilling appears to be small. One study suggests vapour losses account for around 0.18% of fuel purchased.

What was claimed

Refilling your tank when it reaches half-full will reduce the amount of petrol which evaporates within the tank.

Our verdict

Fuel tanks are sealed systems which means that the amount of fuel within a tank shouldn’t affect the amount which evaporates. Regardless, filling up more regularly would require more trips to fill up, which would need to be balanced against any claimed benefit.

What was claimed

Filling your car immediately after a delivery to the petrol station will result in fuel full of dirt and debris.

Our verdict

False. Petrol pumps are fitted with filters to prevent this happening.

A series of tips published in a Facebook post on how to save money while filling up your car are misleading as they would have at most only a negligible impact on the cost of driving. 

The post recommends:

  • purchasing petrol in the early morning when the temperature of the ground is at its lowest 
  • pumping petrol at a slow speed in order to minimise the amount lost due to vaporisation 
  • filling up your tank when it is half full

Similar claims with almost identical wording have been circulating online for at least 15 years and featured in a fact check at Snopes in 2007. They are enjoying a resurgence in the face of rising petrol prices and concerns about the cost of living. 

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Ground temperature

The post says: “Remember that all service stations have their storage tanks buried below ground.. The colder the ground the more dense the fuel, when it gets warmer petrol expands, so buying in the afternoon or in the evening your litre is not exactly a litre.”

It is true that fuel expands as it becomes warmer and that a litre of cold petrol or diesel will weigh more than a warm litre of the same. Tests conducted by Consumer Reports, an independent US product testing organisation, found that when petrol rises from 60 to 75 degrees fahrenheit (15.5 Celsius to 23.8C), its volume increases by 1%. 

However, petrol stations store their fuel in large tanks deep underground. The depth of the tanks means they are isolated from changes in temperature above ground, so the fuel they contain stays at a stable temperature. 

Consumer Reports conducted further tests at a filling station over the course of a sunny day when the air temperature varied by up to 12 degrees fahrenheit (6.6C). These showed that the temperature of the fuel in the underground storage tank remained constant, regardless of the temperature above ground. 

The testers noted that fuel contained in the hoses of the pump may be subjected to greater changes in temperature, but in a busy petrol station where cars are filling up one after the other, the difference will be negligible. 

Insurance provider Provident also states that “fuel is stored underground in non-metallic, double-walled tanks at the vast majority of petrol stations, which keeps the fuel at a steady temperature throughout the day" and suggests the potential savings of filling up in cooler temperatures would amount to a penny or two per tankful, at best. The AA has also debunked this myth.

Some filling stations in the UK may use standard temperature accounting, whereby the amount dispensed is adjusted based on the temperature to eliminate any variability caused by fuel being at different temperatures.

Pumps at petrol stations are tested regularly to ensure accuracy and will always deliver the same volume of fuel regardless of the temperature. By law, pumps are allowed to deliver between 0.5% less or up to 1% more than the amount indicated on the display. 

This means that depending on the individual pump and regardless of temperature, for every 50 litres of petrol you purchase you may receive as little as 49.75 litres or as much as 50.5 litres. 

Vapour recovery

The post claims that fuelling a car more slowly will lead to less petrol being lost through vaporisation, which is when the liquid fuel turns to gas and escapes into the atmosphere.

Most modern petrol pumps are fitted with devices to collect vapours and return them to the underground storage while nozzles are inside a petrol tank, to prevent the vapours being released into the atmosphere. 

This means that, in theory, drivers could be charged for fuel that is pumped but subsequently returned to underground storage tanks via vapourisation.

At least one study found that the rate at which a petrol tank is filled does affect the amount of vapour produced, meaning more fuel could turn to gas and escape.. However, other studies suggest that the amount of fuel lost in this way is likely to be minimal.

In another study, researchers from Thailand placed a small amount of petrol into a test tank and shook it around to turn it into vapour, then added fuel to the tank at a constant rate. They found the loss averaged 40g for every 30 litres of petrol, or 1.3 grams per litre. 

One litre of petrol weighs 720 grams, so the study suggests that the rate of loss averages out at 0.18% of the total fuel purchased. This means for every £100 spent on fuel, as much as 18 pence could be lost as vapour. 

The release of petrol vapours is associated with a number of environmental and health problems, due to their effects upon local air quality; formation of ozone and photochemical smog; and atmospheric warming and climate change. Petrol stations have strict limits on the amount of vapour they are allowed to escape. 

Evaporation

The post suggests that keeping your fuel tank at least half-full at all times will reduce the amount lost through evaporation. 

However, modern vehicles are fitted with systems that ensure that, once the petrol cap is sealed, any evaporated fuel is captured and made available for use in the vehicle. Although older vehicles once vented vapour to prevent a build up of fumes, this is not the case with more modern cars.

Rod Bennett, an automotive engineer and industry consultant told Full Fact that a fuel tank is a “sealed system”. 

He said: “ As fuel is consumed the void at the top of the tank does not fill with air but remains sealed and becomes a slight vacuum. This is why, when the fuel tank is fairly low, there will be a slight hiss when the cap is removed.”

In any case, utilising this tip would also mean that a driver would have to make more frequent trips to the petrol station which, unless they were combined with existing journeys, would lead to extra fuel consumption. 

Fuel saving tips

The post ends by warning consumers not to purchase their fuel after a fuel delivery to the station because of the risk of dirt stirred up during the delivery making its way into your fuel tank. This would not be the case though as both the underground storage tanks themselves are fitted with filters to prevent this from happening. 

None of the tips in the Facebook post appear featured on a list of money-saving recommendations published by the RAC motoring organisation which says that gentle acceleration, remaining within the speed limit and keeping your car properly maintained are among the most important factors for achieving more miles per gallon. Similar tips are provided by the consumer organisation, Which?

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