The Clever Homeowner is reader-supported. When you buy through links on our site, we may earn an affiliate commission at no additional cost to you.

Is It Okay To Mix Old Gas With New Gas? (Or Does It Go Bad?)

mixing old gas with new gas

Have you ever had gas left in a can in the garage over a year or two, and wondered if you can still use it? I have, and I needed to find out if it was still good. Let’s take a closer look at what I found so that you can avoid any potential issues.

Does Gasoline Go Bad?

While gasoline maintains its integrity over months or even years, these petroleum products do break down over time. It can also interact with the storage container. Other environmental factors also influence this fuel source’s composition.

If crude oil lasts for millions of years, why would gasoline go bad? That’s because the distilling process has altered gas by changing its composition.

The petroleum-based product separates into components through fractional distilling. That gas distilling process creates a mixture that is made mostly of organic compounds in the form of hydrocarbons. Unfinished gasoline mixes with a variety of additives that further change the chemical composition.

That’s why finished gasoline lacks the longevity of crude oil.

Read More: What To Do With Old Gas in a Gas Can

The Problem With Evaporaton

Organic compounds become a problem for gasoline stored for too long. These compounds are lighter and begin to evaporate over time.

The loss of hydrocarbons alters the efficiency of the fuel in today’s sensitive engines. Your car’s engine just may be unable to run on that older gasoline.

Other components in finished motor gasoline also evaporate. Each additive will have a rate of evaporation that is unique. Over time, these rates change the overall composition of the fuel you have in the tank.

Does Temperature Effect Gasoline?

It can, but usually not in a direct way. Diesel can begin to gell at extremely cold temperatures. Manufacturers often combat this by making blends for summer and winter respectively.

The additives used to make a winter blend will not run as efficiently during warmer months. They use lighter hydrocarbons that are more volatile. That allows them to ignite easier during cold weather, but they will evaporate quickly during hot weather conditions.

Conversely, summer blends use heavier organic compounds that are less volatile. These have more difficultly igniting during winter conditions.


Humidity changes how gasoline performs. Combustion lasts longer while the burn rate reduces. It will increase the chance of engine knocks.

And as you may already know, water does not mix with gasoline. If water contaminates the fuel, it will settle at the bottom of the container with the gas sitting on top of it.

Once the gas burns off, the water can cause damage to your internal combustion engine. Water can also freeze in a tank or gas lines, causing blockage problems.

Mixing Gasoline With Air

Gasoline that makes contact with oxygen can also begin to gum up. Some hydrocarbons evaporate, while others interact with the air. That interaction creates the solids that can clog lines (“gum” them up).


Today’s gasoline in the North American market contains up to 10-percent Ethanol. This compound bonds with water. It can draw moisture into a container from the air, especially in humid conditions. This is not a good thing.

That process can eventually build into the problems discussed earlier regarding water.

How Long Does Gasoline Last? The Bottom Line

Many sources, including J.D. Power, give regular gasoline a shelf life of up to six months before it begins to degrade. Diesel can last up to a year before it begins to breakdown. They also note that Ethanol will begin to evaporate and change within one to three months.

Can You Tell When Gas Is Bad?

Not really. You can label any storage containers to let you know how long the fuel has sat at your home. Using the guidelines above, you will have an idea when the gas is likely to start breaking down.

Engine performance can be a sign of “bad” gas, but the symptoms can also be a result of other issues.

Storing Gasoline Properly

You can provide the best possible environment to help extend the time it takes before gasoline begins to deteriorate.

To start with, store gas in a proper container. Various metal and plastic tanks are available that are made specifically for fuels.

Make sure that the container is airtight. That will prevent the hydrocarbons from mixing with the air. It reduces evaporation and prevents gumming of the gas.

Minimizing humid conditions will prevent moisture build-up, especially in products with Ethanol.

Can You Mix Old Gas With New Gas?

It is possible, but you will need to mix the new gas at a proper ratio with the old gas. That will allow the engine to fire at the best possible rate. You can also continue to top off a tank as you burn the mixture, providing more fresh gas to the mix.

Also, make sure that the old gas does not contain water or other impurities that would damage the combustible engine.

Use a proper container to transfer the gasoline to the tank you are mixing. Use a funnel to limit spills and speed up the process. Less exposure to the air and humidity is beneficial.

Mix smaller ratios of old gas

You do not want to mix in small amounts of new gas into large volumes of old fuels. That will not dilute the older gasoline enough to make a difference. Instead, try to add smaller amounts of old gas with fresh gasoline.

For smaller engines, like those on a lawnmower, you can top an old tank of gas off if it is half full or less. The mower should fire, and you can burn the mixed fuels off while cutting the grass.

Vehicles can be a bit more tricky due to the larger volumes of gasoline involved. Determine the vehicle’s tank capacity (found in the Owner’s Manual). The range will be from 9 to 16-gallons for most automobiles.

A smaller tank of 9 or 10-gallons that is 3/4 full can handle mixing in a half-gallon of old gasoline. Medium-sized tanks around 12-gallons can mix 3/4-gallon of old fuels, while the large 16-gallon tanks can take up to a gallon of old gas if it is nearly full.

Mix the fuels in a tank that connects to an engine that you plan to use. That burns up the old gas. You won’t be storing the mixture that way, preventing an even larger volume of old gas in the future.

Are There Ways To Salvage Old Gas?

Yes, there are. The real question is, do you have the time or money to do it? For small amounts of fuel, it is probably not worth it. Larger volumes, such as those in full vehicle gas tanks, might be worthwhile.

You also need to consider why the gas is old or “bad.” Gasoline that is two months old or less should not need reconditioning. If it is over two-years-old, it might be beyond your ability to salvage.

Gumming from oxidation

You can filter gasoline that has begun to gum. The filter will remove the build-up within the fuel, leaving gasoline that should not clog your lines.

Moisture contamination

Smaller amounts of gasoline can be decontaminated by hand. Pour the water infused gasoline into a jar and let the water settle to the bottom. You can then pour the top layer of fuel into another container.

Larger volumes will prove too difficult for this method. Commercial products are available that leech the water from the gasoline. These products are also an option if you do not want to separate smaller quantities by hand.


The fuel blends and additives can separate into layers that do not mix. You will need to replace the hydrocarbons that evaporated. Commercially available additives will allow the fuel layers to mix, restoring the gasoline to its previous condition.

How To Dispose Of Old Gas

If the gas is beyond reconditioning, and it is too old to mix with new gasoline, you will need to dispose of it. Fuels are combustible and dangerous for the environment, so you need to get safely rid of them.

First, you need to check your local city ordinances and the laws of surrounding areas. These rules will let you know how (and where) to legally dispose of old gasoline.

Next, you can use a site like Earth911 to find local recycling centers that might take the fuel. If a recycling center is not available, you can contact waste disposal sites, or possibly the local fire station. A garage or vehicle service center might take the gas as well. If you are having trouble finding a location, contact your local government, and they should be able to help.

Once you know where to take the old gas, you will need to put it into a proper storage container for transportation. Commercially available gas cans are safe to use as long as you make sure that they won’t spill as you drive.

Call ahead to make sure the facility is ready and able to accept old gasoline and follow any additional instructions that they require. If you are lucky, you might find a local facility that will pick up the old gas for a small fee.

how often to pump septic tank

How Often To Pump a Septic Tank (Frequency Chart)

old gas in gas can

What Do You Do With Old Gas In A Gas Can (How To Dispose of Old Gasoline)