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As stuff gets warmer, it expands. That’s why bridges, freeways, and sidewalks have expansion joints. The gaps provide room to expand into on hot summer days.

Gasoline does the same thing. As it gets warmer it takes up more room. But “bigger” gasoline doesn’t power cars father. The amount of energy in gasoline depends on its mass, not its volume. Since we buy gasoline by the gallon, are we getting ripped off when we buy hot gas?

To answer that, we need to answer a couple questions:

1. How much does gasoline expand?
2. Do we buy hot gasoline?

Does Gasoline Expand Much?

Gasoline’s volumetric coefficient of expansion is 950 X 10^-6 °C^-1. To put that in context, the graph below shows the coefficient of expansion for some common materials (click for a larger view).

Impressive (or depressing) indeed. Notice that gasoline expands 26 times more than concrete.

But what does it mean? If you have a gallon of gas at 60°F and increase its temperature to 90°, its volume increase by 1.58 percent, or about one-quarter cup. Suppose you buy 20 gallons of 90° gasoline on the way home from work. The next morning when the gasoline has cooled to 60°F, it takes up only 19.69 gallons. Assuming the price was $4.00 per gallon (yeah, right…), you effectively paid $4.063 by buying hot gasoline. Put another way, you paid an additional $1.25 to fill your 20 gallon tank.

Do We Buy Hot Gasoline?

Does gasoline get that hot? Certainly it does in a tanker truck rolling through the heat of summer. But what about when it’s stored in a gas station’s tank buried underground? According to multiple sources, the temperature underground is relatively constant night or day. Buying gas in the morning or evening can’t make a significant difference.

What can make a difference is not buying gas right after a tanker dumps its load of hot gas, before the gas has had a chance to cool down.

What could make a bigger difference is watching how your drive: keep your speed down, don’t accelerate too quickly, or better yet (at least for saving gas), decrease your driving.

What are you doing to save your gas money?