Debunking a Legendary Gas Pump Myth

Find out if filling your gas tank during cooler temperatures will help you get the most for your money or if it’s a legendary myth.

A few weeks ago, based on some (admittedly cursory) research, we posted this tweet:

#DidYouKnow that gas pumps deliver more fuel during cooler temps? As summer heats up, get more cool mileage tips: http://bit.ly/9vQl49

We then went about our day feeling confident in the fact that we’d provided our followers with a useful, money-saving tip that could be put to good use.

And we might have gone on thinking this forever were it not for @ChrisSheehy who challenged our accuracy with a tweet of his own:

Better check your facts on this one – it’s a legendary myth : ) RT @esurance: …gas pumps deliver more fuel during cooler temps…

Uh-oh. Not wanting to be responsible for the dissemination of untruths (let alone urban mythology), we decided to do a little digging.

And so we dug. And dug. As it turns out, this is a much-debated gas pump myth across the interwebs, and we found a lot of smart-sounding arguments on both sides of the fence. Some sources claimed that yes, you do get more for your money when you buy cooler gas. Others pointed out that the average temperature of gas kept in underground tanks fluctuates very little and so your savings would be negligible. And others still argued that gas is warmer when it first reaches the station from the refinery, and so that’s when you should avoid buying it.

As you might imagine, all our digging made us slightly befuddled and more determined than ever to get to the bottom of the issue — which we did. Finally.

The truth behind the gas pump myth

Here’s the bottom line. At 60 degrees Fahrenheit, a gallon of gas measures 231 cubic inches. But the same gallon at 80 degrees Fahrenheit measures 233.7 cubic inches (according to the New Hampshire Department of Agriculture, Markets & Food). So theoretically, if you pump a gallon of gas at 80 degrees, you’re paying for 233.7 cubic inches, but once that gas cools to 60 degrees in your tank, you only have 231. That’s just simple math and physics (wink).

However… (and here comes the important part) underground fuel tanks essentially function as huge thermoses. If the gas is warm when it’s pumped into the tank, it will likely stay warm for a while. Similarly, once the gas cools in the tank, it will remain at that cooler temperature regardless of how things may be heating up above ground.

Additionally, due to something called thermal inertia, ground temperatures at depths below 4 feet remain (roughly) between 50 and 60 degrees, providing further insulation for the thermos-like fuel tanks.

What all this math and science boils down to is this: The temperature of the gas at your local pump varies little throughout the day. And that, of course, leads us to conclude that whether you’re wearing a wool sweater or Bermuda shorts when you fill your tank, you’ll get roughly the same volume of gas for your dollar (or 3).

So there you have it … we stand corrected. But we know a lot more about gas temps and volume now than we ever did before and are all too happy to share our newfound wealth of info with you.

Oh, and thanks for the tweet, Chris. You had us at “legendary myth.”

Related links

CBS MoneyWatch article
Consumer Reports story

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