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. As a result, consumers end up paying a “hot fuel premium” every summer in the neighborhood of $4 billion.
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 340 million people in the United States. Assuming that about half of them drive, the $4 billion hot fuel premium costs each driver about $23 per year. That probably doesn’t amount to even a half tank of gas per year for the average driver. Then again, that 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. That’s because the Aloha State already requires retail stations to install automatic compensation devices on all its gas pumps.
Then again, if moving to Hawaii 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, waiting until the outside air temperature is as cool as possible will not result in any significant gasoline savings. But at least you can be assured when you pull up to the pump you’ll get what you paid for.
Photo Credit: stock photo
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?
Len Penzo says
In a word: yes.
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.
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 (59 F). 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!
The fact is gasoline is stored underground in double walled tanks that act like a big Thermos bottle. There is no measurable temperature change in the gasoline over a 24 hour period. It’s been tested. The gasoline in the transfer piping does heat up or cool off with ambient conditions but unless you are the only customer those lines are flowing so much the fuel doesn’t have time to change temperature. At least that’s what I deduced when I researched this. I’m a licensed professional chemical engineer who produced gasoline for my entire 38 year career.
Len Penzo says
That makes sense, Steve. Thanks!
And I also suspect that the thermos effect of those large underground tanks probably becomes moot for stations that get daily or even multiple deliveries per day.
No matter how you slice it, while the opportunity for savings exist, they aren’t going to be significant. That being said, I’m sure there are a few hyper-frugal folks out there who still care. 🙂
Thank goodness that pump is accurate to 3 decimal places. I want exactly 1.000 gallon, not 1.001 or 0.999 gallons. 1/8th of 1 percent? I like avoiding all the benzene and volatiles coming off the nozzle in hot afternoon. So agree to fill up in the morning….and grab a cup of joe.
Len Penzo says
LOL! 🤣 🤣 🤣
I found that filling up in the morning means an uncrowded gas station. I don’t feel stressed, and I can tell if Len is trying to slip a fruit cake in my truck.
Len Penzo says
LOL! Bill, your random fruit cake jokes are the gift that keep on giving. (Just like a real fruit cake.)