How Fast Does Gas Go Bad Just Sitting In Your Car?

In what may be the quintessential “first world problem,” consider that rich people with large car collections have to do constant maintenance to mitigate the effects of expiring gasoline. Opportunities for the average person to worry about old gas are much farther and fewer between, and usually involve lawnmowers or portable generators. But when you have a car that might sit for a year (or years), you’d better exercise due diligence to prevent that fuel from damaging engine components, leaving gummy residue that acts like plaque in arteries.

In general, non-ethanol gas lasts anywhere from three to six months in your gas tank. As for ethanol blends like E10 (10% ethanol, 90% gas), you’ll be shocked to hear that Minnesota Corn, whose mission is to “maintain and develop markets for corn and corn co-products,” says E10 will last at least six months, and longer with good storage. Meanwhile, market research firm J.D. Power (you know, the longtime award-giver and name most heard in Chevy ads for a while) says ethanol fuels can lose combustibility in just one to three months. 

Also, while modern cars are designed around gasoline having a certain ethanol content in fuel, older cars with untreated rubber and plastic seals can apparently suffer serious damage. So, if you’re storing gas for your Ford Model A or Bugatti Royale, perhaps use straight gas with no additives. 

Should you want to stockpile gasoline like a dragon keeps gold, remember that no matter what you do, gas won’t last forever, but you can preserve its freshness for a surprisingly long time. First, store it in airtight, fuel-rated plastic containers with as absolutely little air in them as possible. Then, put those containers in a cool, dry place. If you have access to, say, the Subterranean Ultra Secure Supercar facility and its nitrogen-only atmosphere that requires robot workers, so much the better. Don’t expose your gas containers to extreme heat or cold, either. Gas likes a stable temperature; otherwise it will evaporate and oxidize faster.

If you are one of those wealthy people with a vast car collection, you can take the cars out for a short spin every few weeks. Then, top off the tank with fresh gas, which simultaneously reduces oxygen levels in the tank to prevent evaporation and limits room for moisture buildup. I’m available day or night to come drive the cars, a sacrifice I’m willing to make. I’ll make sure to change the oil so it doesn’t go bad, either.

You can also add fuel stabilizers that will keep your gas from evaporating; then you may be able to store fresh gas for up to two years. According to tests from FortNine, the top performers are STA-BIL and K100. Still, even if you do use stabilizers and store your gas in a dehumidified cave, always check your gas before using it. You’ll know if your gas has gone bad if it smells stale or sour, transitions from a lovely amber color to a dark orange, or thickens like syrup, then it’s no good. No word on whether the flavor changes, though.

According to Sunoco, plain 87 octane lasts three months before degrading, but 93 octane can last nine months before any appreciable degradation occurs. Higher octane ratings equal more-stable fuel. Race gas, thanks to its extreme refinement and octane-preserving chemicals, can last two years or more with careful storage. Uncareful storage kills it faster, because octane boosters like methylcyclopentadienyl manganese tricarbonyl (MMT) break down after mere minutes in sunlight.

Gasoline manufacturers need to simultaneously increase knock-preventing octane, keep fuel affordable, and tailor fuels to prevent vaporizing in various climates, and these goals are interwined. Relatively inexpensive additives can have low boiling points, such as butane, which boils at 32 degrees. Racing fuel has additives that boil at higher temperatures (80 degrees or more) but are pricier. 

To see how various fuels can stay stable in temperature extremes, manufacturers test fuel blends by sealing them in containers and heating them to 100 degrees. This produces a Reid vapor pressure (RVP) measured in pounds per square inch (psi), with higher pressures equaling higher volatility. This is why higher-RVP fuels, around 12 psi or more, are better suited for cold weather while the opposite is true in warm weather. Federal law mandates an RVP pressure of 9 psi for fuel sold between June 1 and September 15, aka, “summer season,” to prevent gas evaporating and releasing ground-level ozone. Individual states can be even stricter, such as California, which limits RVP pressures to 7 psi. 

So, when you wonder aloud why gas prices are so high, it’s not just fuel taxes and delivery costs, though they’re huge factors (remember when a remote gas station in the Mojave desert wanted $8.59 a gallon?). Equipment used for RVP testing is expensive, and the costs get passed to you.