Popular advice on how to fill cars with gas does not offer great savings

A series of tips published in a facebook post on how to save money by filling up your car’s tank are misleading, as they would, at best, have a negligible impact on the cost of driving.

The post recommends:

  • buy gasoline early in the morning when the ground temperature is lower
  • pump gasoline at low speed to minimize the amount lost due to vaporization
  • fill the tank when it is half full

Similar claims with nearly identical wording have been circulating online for at least 15 years and appear in a fact checking in snopes in 2007. They are enjoying a resurgence in the face of rising gas prices and cost-of-living concerns.

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ground temperature

The post reads: “Remember that all gas stations have their storage tanks buried underground. The colder the ground, the denser the fuel, when it warms up the gasoline expands, so buying in the afternoon or at night your liter is not exactly a litre.”

It is true that the fuel expands as it heats up and that a liter of cold gasoline or diesel will weigh more than a hot liter of it. Tests by Consumer Reports, an independent US product testing organization, found that when gasoline goes above 60 to 75 degrees Fahrenheit (15.5 Celsius to 23.8C), its volume increases by 1%.

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

Consumer Reports conducted further tests at a gas station over the course of a sunny day when air temperatures varied 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.

Testers noted that the fuel contained in the pump hoses may be subject to greater temperature swings, but in a busy service station where cars fill up one after another, the difference will be negligible.

Insurance provider Provident also states that “fuel is stored underground in double-walled, non-metallic tanks at the vast majority of service stations, which keeps the fuel at a constant temperature throughout the day” and suggests the savings potential to fill in coolers. temperatures would be equivalent to penny or two per full tank, at best. The AA has also debunked this myth.

Some service stations in the UK may use standard temperature accountingso the amount dispensed is adjusted based on temperature to eliminate any variability caused by different fuel temperatures.

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

This means that depending on the individual pump and regardless of temperature, for every 50 liters of gasoline you buy, you can receive anywhere from 49.75 liters to 50.5 liters.

Vapor recovery

The publication claims that fueling a car more slowly will result in less gasoline being lost to vaporization, which is when liquid fuel turns into gas and escapes into the atmosphere.

Most modern fuel pumps are equipped with vapor collection devices and return them to underground storage while the nozzles are inside a gasoline tank, to prevent the vapors from being released into the atmosphere.

This means that drivers could theoretically be charged for fuel that is pumped out but then returned to underground storage tanks by vaporization.

At least one to study found that the rate at which a gas tank is filled affects the amount of vapor produced, meaning more fuel could convert 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 He placed a small amount of gasoline in a test tank and stirred it into vapor, then added fuel to the tank at a steady rate. They found that the loss averaged 40g per 30 liters of gasoline, or 1.3 grams per litre.

A liter of gasoline weighs 720 grams, so the study suggests that the loss rate averages 0.18% of the total fuel purchased. This means that for every £100 spent on fuel, up to 18 pence could be lost as steam.

the release of gasoline vapors it is associated with a number of environmental and health problems, due to its effects on local air quality; ozone formation and photochemical smog; and atmospheric warming and climate change. Gas stations have strict limits on the amount of vapor they can escape.


The publication suggests that keeping the fuel tank at least half full at all times will reduce the amount lost to evaporation.

Nevertheless, modern vehicles they are equipped with systems that ensure that, once the gas cap is sealed, the evaporated fuel is captured and made available for use in the vehicle. Although older vehicles once steam ventilated to avoid the accumulation of fumes, this is not the case in most 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. That’s why, when the fuel tank is quite low, there will be a slight hiss when the cap is removed.”

In any case, the use of this advice would also mean that the driver would have to make more frequent trips to the gas station which, unless combined with existing trips, would entail extra fuel consumption.

Tips to save fuel

The post ends by warning consumers not to purchase their fuel after a fuel delivery at the station due to the risk of debris being stirred up during delivery and reaching the fuel tank. However, this would not be the case as both underground storage tanks are equipped with filters to prevent this from happening.

None of the advice in the Facebook post appears on a list of money-saving tips published by the CAR motoring organization that says smooth acceleration, staying within the speed limit, and maintaining your car properly are among the most important factors in getting more miles per gallon. Similar Tips are provided by the consumer organization, which one?

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