Have you ever heard the advice to fill up your cars‘ gasoline first thing in the morning? This fuel-saving myth posits that because gasoline shrinks in the cold, you’ll get more gas for your money in the morning. The truth is that gasoline does shrink when it gets colder, but it is unlikely it changes temperature much in the gas station’s underground storage tanks.

Gasoline expands and contracts with the temperature

According to Snopes, gasoline expands or contracts 1% with every 15 degrees of temperature change. If a gallon of gas gets colder, it actually shrinks slightly and takes up less than a gallon of space. Likewise, if that gallon of gas gets hot, it will expand.

Legislators know that this means not every consumer gets a fair shake. Most gas pumps are set to measure gallons of gas at 60 degrees Fahrenheit. But in Hawaii, the pumps are calibrated for 80 degrees. Without this recalibration, a Hawaiian gallon of gas would shrink to less than a gallon at lower temperatures.

Other legislators have proposed switching to gas pumps that use a technology called Automatic Temperature Compensation (ATC). These gas pump measure the temperature of the gasoline and adjust their volume measurements accordingly.

Is it worth pumping gasoline early in the morning?

It probably isn’t worth your effort to pump your gasoline very early in the morning to save money. Though gasoline does contract a bit when it gets cold, morning gasoline is probably the same temperature as nighttime gas–perhaps warmer.

The fact-checkers at Snopes point out that gasoline is stored in huge, underground tanks, deep beneath gas station parking lots. These tanks, the dirt and concrete around them, and the sheer amount of gasoline they contain, all work together to insulate the gasoline. It is unlikely that the gasoline’s temperature will be any lower at a certain time of day.

It may actually be bad to pump your gasoline early in the morning. If the oil company delivers fresh gasoline to your station in the morning, the fuel straight from the refinery might actually be very hot. Obviously, this gasoline in the underground tanks will stay hot for a while. If you pay for a full tank of this hot gasoline it will later cool down and shrink.

Can cold gasoline save you money?

Some scientists have pointed out that gasoline does not gain more “energy” as it shrinks or expands. Therefore, cold, shrunken gasoline will likely bump up your MPG. Hot, expanded gasoline will decrease it. Therefore, you will get just as many miles/dollar, whenever you fill up.

Finally, even if buying cold gasoline in the early morning successfully got you 1% more fuel at every fillup, that 1% in gas savings would not save you much money. That 1% would translate to $31/year for the average American driver. Is that really worth getting up at dawn every time you need fuel?