Does buying your gas when it’s cold really save you money?

With gas prices skyrocketing and the temperature not far behind (at least here in New England) there is always talk about how to save money on gas. One topic which seems to have solidly hit the mainstream lately is the notion that you can save money by buying your gas when it’s cold. This has herds of people buying their gas on cold days or in the wee hours, but is it worth it? Well, consider the following:

Gasoline, like most things, does expand when it’s warm. According to this study performed by a House committee last year the ratio is around 0.069% per degree Fahrenheit. That means if you are paying $3.75 per gallon you may be paying a full 7.7625 cents more per gallon for gas at 90 degree than you would at 60 degrees. That adds up to about 93 cents on a 12 gallon fill up if the gas is a full 30 degrees warmer.

Now, before you run off in the middle of the night to save your scant dollar on a fill up, consider this: Gas is stored in very large tanks, and often under ground. Next time you’re filling up on a warm day feel the temperature of the hose or the metal parts of the nozzle… You’ll probably find that they’re much cooler than the ambient temperature, so the swing in the temperature of gas will vary much less than the temperature of the air.

So for my money, instead of getting up early and wasting gas with a special trip to the station, consider combining some errands, carpooling, or slowing down a couple MPH on the highway. These changes will not only save you money, but also save gas and reduce emissions.

How does the gas pump shut off

So being a very curious person I can’t stand not knowing how things work. Thanks to the wonderful folks at Google, I can usually find a fairly quick answer. So when I got a burning curiosity to know how gas pumps shut off when the tank in my car is full (and my credit card is empty) I was surprised I couldn’t find a quick answer.

My favorite site for this type of thing is They are very comprehensive on most topics I have pursued there, but have barely more than a paragraph on the topic.

Not being satisfied with my initial findings I moved on to more advanced search techniques (really I just kept trying different search terms in Google.) From there I came up with this article from (you’ll have to page down past the article about sleeping birds.)

The short story on this is basically there is a small vacuum tube that runs down the spout and has an opening near the tip. The pumping gas produces suction on the vacuum tube and when gas gets high enough in the tank to cover the end of the tube it increases the vacuum on the other end of the tube which trips the shut-off.

Think about it this way: if you were sucking air through a straw, then someone put the other end of the straw into a cup of water you would notice that it became harder to suck. With any luck at all, you would stop sucking before you filled your lungs with water, just like the gas pump stops before it fills your shoes with gas.

For a better explanation, here is the article from Another interesting article from Husky describes the technology, as well as goes into more detail as to why it sometimes fails (as well as describing why their nozzles are the best, of course.)

All-in-all a very elegant, low tech solution requiring no electricity, sensors, floats, or any other fancy gadgets; one that has passed the test of time.

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