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Is It Ok to Mix Old Gas with New Gas?

Is It Ok to Mix Old Gas with New Gas?

You may be wondering what happens to the gas in your vehicle if it sits for a long period of time. 

This is a valid concern for people that may store their vehicles during a vacation or over the winter months.

When it comes time to hit the road again, can you just add new gas to your tank and top it off? Can you mix old gas with new gas?

Mixing old gas with new gas is normally not an issue for most vehicles. When you add new gas to a tank that has old gas in it, the two ages of gas will mix and still be usable for your vehicle.

There can be a few minor issues to keep in mind, however, like the amount of old gas you are mixing with new, and just how old the gas actually is. 

Let’s take a closer look at mixing old gas with new gas and whether or not it will damage your engine.

Can Old Gas Be Mixed With New Gas?

When asked, most mechanics will say that old gas in a vehicle is not going to be an issue. 

While the quality of gas does start to drop, it can take many months or even years before it gets to a point where it is completely unusable or dangerous for your engine.

In most cases, gas can sit in your vehicle’s gas tank for a few months before it is considered old. If you have some old gas in your tank and top it off with new gas, there will not be any issues with your engine performance.

However, if you have a tank full of gas that is several months or even a year old, it may not have the same grade and octane balance needed to keep your car’s engine running smoothly. 

In this case, it might be a better option to drain the gas and add fresh, instead of trying to run the car on the old gas.

Will Old Gas Mess Up Your Car?

As gas sits in a gas tank without being used, it starts to degrade. Over the course of a few months, the octane can start to break down and evaporate. Octane is the part of gas that creates combustion which is needed for your engine to operate properly.

While it can take several months before most gas starts to show signs of being old or not being as effective in your engine, there are a few ways you can tell if the gas in your car is older than usual:

  • Engine sputtering. When idling and when at highway speeds, your engine may sputter or cough. This is normally caused by the lower combustion rate in the gas affecting the effectiveness of the spark plugs in the engine.
  • Lack of engine power. Usually, you will notice a lack of engine power. As gas ages, it loses a lot of the combustion qualities it has when new. This can lead to poor performance in your engine which shows itself as a lack of power.
  • Difficulty turning the engine on. While this is not a normal sign, it can still be used in some cases. Normally a car will turn on just fine even if it uses old gas, but sometimes the gas can be too evaporated to run through the system properly.

Some of the long-term engine issues you can encounter when using old gas may include clogged injectors. While this is a somewhat easy maintenance task for yourself or a mechanic to perform, if left unmaintained it can lead to more serious engine issues over time.

Can Old Gas Be Made Usable?

If you are dealing with small amounts of old gas, normally the easiest way to make it usable is by adding fresh gas to it. The gas will mix and the car’s engine and computer systems can help balance the combustion properly as it passes through the engine.

However, if you have a large amount of gas that has sat unused for several months or a year, you might want to try to reverse time and make it more usable. 

Finding the right method will depend on how aged the gas is.

  • Oxidized Gasoline. As gasoline is exposed to air inside a gas tank, it will oxidize over time. This can cause the gas to become thicker. This thick gas will normally be caught by the fuel filter before it has a chance to clog your gas lines. In most cases, you can use oxidized gas normally as long as your fuel filter is working properly.
  • Too Much Moisture. Sometimes moisture or condensation can get into your gas tank when the fuel sits for long amounts of time. When dealing with too much moisture in a portable gas can, you can easily separate the gasoline from the heavier water by using a siphon or hand pump.
  • Improper Layering. Over time, gas will start to separate into different layers as hydrocarbons are evaporated from the gas itself. In this case, you will need to purchase a commercial additive that can help these separated layers mix together properly again.

If you are going to be storing your vehicle for several months such as when traveling on holiday, or when storing it through the winter months, you can add a fuel stabilizer to the gas tank. 

This liquid will mix with the gasoline and help keep it viable even when stored for 12 months or longer.

Using a fuel stabilizer is extremely common with recreational vehicles and boats which may only be used once or twice a year, or perhaps even every other year. Sometimes adding a fuel stabilizer can be a much better option than completely draining the gas tanks.

Related Questions / Frequently Asked Questions (FAQs)

How can I keep gas safe to use for a long time? 

Use fuel stabilizers. These liquid additives can extend the shelf life of gas by two years or longer giving you plenty of time to store your vehicle throughout the seasons.

Can I use old gas in a lawnmower

In most cases, yes. Depending on the age of the gas, your fuel filter will trap any gummed-up components while allowing the rest of the gas to pass into the engine lines.

Will using old gas ruin my car’s engine? 

While some newer cars can suffer from an improper combustion mix in fuel, others can do just fine on aged gas. You may need to replace your fuel filter more often and ensure your gas lines do not become clogged.


In general, using old gas in your car is not dangerous – however, it will depend on the actual age of the gas as well as how it has separated over time. 

Most gas is perfectly safe to store for a few months without any issues, but as it gets older it will eventually start to break down and become less effective as a combustion element for your car’s engine.

If you plan on storing gas in a vehicle’s gas tank or a portable gas can for extended periods of time, consider adding a fuel stabilizer. 

This additive can be found at most automotive supply stores and can extend the life of gasoline for up to 2 years.

Zach Reed

Hi, I'm the founder of! Having owned a wide variety of vehicles in my life, I was astounded at how hard it can be to find answers to common automotive questions. Rather than sit idly, I decided to create this website to help others!