Does Buying Your Gas In The Early Morning Really Save Money?

I’ve already written about the folly of trying to save a few cents per gallon when shopping for gas.

But did you know there are times when you don’t get exactly what you pay for when you fill up your gasoline tank at the local station?   It’s not that the gas stations are doing anything illegal.   But they are taking advantage of the laws of physics in a way that permits them to make a little extra money at your expense.

Now, for those of you who regularly ditched your high school physics class, let me give you a quick primer on why this is so.   😉

Gasoline expands when temperatures rise, but the energy content of gasoline is directly related to its weight, not its volume. The end result of this expansion results is less energy per gallon.

Now, it’s absolutely true that gasoline retailers adjust for hotter weather when they buy fuel at the wholesale level. But it’s also true that the very same retailers (at least in the United States), knowingly refuse to make the same warm-weather adjustments when they sell their gasoline to the public on hot days. The result of this is consumers end up paying a “hot fuel premium” every summer in the neighborhood of two billion dollars.

This tends to get a lot of nerds who actually enjoyed their physics classes really spun up. But should they really be that upset?

There are roughly 300 million people in the United States. Assuming that about half of them drive, the two billion dollar hot fuel premium costs each driver about $16 per year. That probably doesn’t amount to even a half tank of gas per year for the average driver, although it ultimately depends on the type of car you drive, and the cost of fuel.

Still, if you want to ensure you avoid the summertime hot fuel premium you can move to Hawaii, which  already requires retail stations to install automatic compensation devices on all its gas pumps.

For those who don’t live in the Aloha State, they  may be happy to know that Costco recently announced that they have agreed to install the same type of gasoline compensation devices at stores in 14 states. Those states are Alabama, Arizona, California, Florida, Georgia, Kentucky, Nevada, New Mexico, North Carolina, South Carolina, Tennessee, Texas, Utah, and Virginia.

Then again, if  a trip to Costco is either impractical or a non-option, the effects of the hot fuel premium can be minimized by filling up in the early morning when temperatures are coolest.

True, the Costco decision will not result in any significant gasoline savings, but at least you can be assured that when you pull up to the pump you will get what you paid for.

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  1. 1

    Vince says

    We should submit this to the mythbusters. Install one thermometer inside the underground gas tank, one inside the nozzle and one for ambient air temp and graph the differences, if any.

    For the temperature of the gas at the nozzle, I expect the temp to decrease quickly as cool gas absorbs heat from the hotter hose and pipes.

  2. 2

    BigGeek says

    It’s true…According to the Canadian Petroleum Products Institute, “it is estimated that a given volume of gasoline will increase or decrease approximately by 1/8 of 1% for every degree of temperature change.” Might be negligible…or not if you have large temperature swings where you’re buying gas.

    As far as I know, US pumps sell gas based on volume at ambient temperature without temperature correction. There are also vapour recovery devices on the nozzles that suck back gasoline vapours from your tank AFTER you’ve already paid for it. Grr.

    On Canadian pumps, volumes are corrected to 15 degrees Celsius. Actually, that works in our favour only on days above 15 C, and we get gouged on volume compared to non-corrected ambient sales most of the year when it’s colder than that!

  3. 3


    I agree with all the physics with one exception. Considering that the storage tanks are underground. Shouldn’t the temperature of the gasoline in those tanks stay fairly constant?

    • 4

      Len Penzo says

      Yes, it would, Jose. However, the first several gallons being pumped into your car’s gas tank come from the above-ground pump dispenser. After that, the remaining gas is being drawn from the underground tanks.

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