Driving Today: When Should You Buy Gas?

When Should You Buy Gas?
Tom Ripley

Rumor has it that gas pumps deliver more fuel when it is cooler outside.

This suggests you should buy your gas early in the morning rather than in the heat of the afternoon.

Extending the thought still further, not only do some sources claim that you get more for your money when you buy cooler gas, but they also recommend that you not buy gasoline that has been recently delivered to a filling station because that recently transported gas is warmer than the gas in underground tanks.

The story has all the trappings of a major urban myth, so Esurance, an online insurance provider with an interesting blog, sought to get to the bottom of this. And here’s the bottom line: At 60 F, a gallon of gas measures 231 cubic inches, but the same gallon at 80 F measures 233.7 cubic inches, according to the New Hampshire Department of Agriculture, Markets and Food. So theoretically, if you pump a gallon of gas at a temperature of 80 degrees, you receive 233.7 cubic inches, but once that gas cools to 60 degrees, you only have 231.

That might seem like you are being shorted a little and force you to recall a noted “Seinfeld” episode, but there are other factors at work.

First, the underground tanks in which gasoline is stored at the gas station 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, but once the gas cools in the tank, it will remain at that cooler temperature regardless of how things are heating up above ground. More important, due to a phenomenon called thermal inertia, ground temperatures at depths below 4 feet remain between roughly 50 F and 60 F year-round no matter what the ambient temperature is above ground.

This provides further insulation for the fuel tanks, so the temperature of the gas at your local pump varies little throughout the day. That, of course, led Esurance to conclude that whether you’re wearing a wool sweater or Bermuda shorts when you fill your tank, you’ll get very close to the same volume of gas for your dollars.