Does Keeping Your Fuel Tank Half Full Actually Prevent the Gas From Evaporating?

An email full of gas-saving tips is going around. It is supposedly from a longtime petroleum pipeline engineering giving gasoline buyers some tips to improve their gas mileage and get more fuel for their money. One tip is to never let your tank fall below 1/2-full because the gasoline will supposedly evaporate faster as your tank fills with air. Unfortunately, this gas-saving tip is probably bogus, but you do have other options to increase your mpg.

Does gasoline evaporate more quickly when your fuel tank is empty?

Most of the gasoline in your fuel tank is in liquid form. Some of it is suspended in the air in your tank in vapor form. The amount of vapor gasoline depends on the amount of air in your tank and the temperature. But even in a nearly empty tank, the ratio of liquid gasoline to vapor gasoline is stable and no gasoline “disappears.”

Closeup of a driver's hand pumping gas into the half full tank of their car.
Pumping gasoline | Dawn McDonald via Unsplash

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Not only does the amount of vapor gasoline in your tank stay relatively stable, but every modern car is equipped with a vapor recovery system. This is because the EPA regulates how much gasoline vapor an automaker can allow to escape. This system vents the air out of your tank, captures any gasoline vapors, and pipes them to the engine to be burned.

Even if gasoline were to evaporate more quickly in a nearly empty tank, those fuel vapors wouldn’t be “lost” but would be burned alongside the liquid gasoline.

Could keeping your gas tank half full save you money?

There’s a rumor going around that keeping your gas tank more than half full prevents the gasoline you do have from evaporating and saves you money. It is simply not true that your remaining gasoline evaporates when your tank nears empty. But the fact-checkers at Snopes ran the numbers anyway and found that, even if it were true, your money savings would be negligible.

Remote gas pump in front of a mountain range.
Gas pump | Khamkeo Vilaysing via Unsplash

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The experts at Snopes looked up how quickly gas can evaporate in storage. The publication found that companies selling gasoline vapor capturing systems for fuel storage tanks claim that 2% of fuel can be lost in storage.

If your car’s gasoline tank lost 2% of its fuel per year and keeping your tank more than half full could eliminate 100% of this loss, the average motorist would stand to save…$62/year. Would that be worth stopping twice as often to fill up for gasoline–for an entire year? Snopes adds that you would lose seven hours of your life filling up that often. Probably not worth it.

How do you save money on gasoline?

Car parked at an empty, night-time gas station.
Gas station | Jean Christophe Gougeon via Unsplash

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With skyrocketing gasoline prices, most motorists are looking to save a little at the pump. While keeping your tank half full may not be the answer, you do have options.

One thing you have some control over is the state of your car. Keeping your tires inflated to the upper end of your manufacturer’s suggested pressure is a great way to reclaim a couple of mpg. Also, make sure your wheels are properly aligned and your engine is tuned up. If you have any filters that need replacing, swap those too. And finally, do you have heavy and wind-resistant accessories such as roof racks? Make sure to pull those off when you aren’t using them.

You can also control your driving habits. “Hypermilers” who drive for maximum fuel mileage slow down both their starts and stops. They only use A/C when absolutely necessary, and they even plan routes based on avoiding stops and starts.

Find out whether pumping gasoline when its cold saves money or learn more about how to save fuel in the video below:

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