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What Happens When You Mix Regular and Premium Gasoline?

Updated: Jun 28, 2023


Gasoline Pumps

You're driving your car around and you realize your gas gauge is getting low. What should you do? Should you save a little money by filling up on regular instead of premium? Or are you better off with the pricier premium fuel? Do the savings really outweigh the extra cost?


When it comes to mixed fuels, the answer is that it depends. You cannot use regular-unleaded gasoline with premium fuels, however, if you have a car that requires premium fuel and accidentally put regular gasoline in your tank, you may experience engine problems. How can you identify if this is happening? If your car feels like it's struggling to accelerate or doesn't seem as powerful as normal, there's a possibility that you have put regular gas in a tank meant for premium fuel. If knocking or detonation occurs at lower RPMs (roughly 2,500 rpm or less), then this is an indication that you are running lean due to mixing too much regular-unleaded gasoline with a full tank of premium fuel.


Engine Running Rich Vs. Running Lean


Running an engine rich: is a detrimental mistake for many drivers. The ignition timing, fuel mixture, and overall temperature of your engine need to be carefully balanced in order for the internal combustion process to run smoothly. When you drive with an overly rich mixture, carbon buildup occurs on your pistons and exhaust valves, which reduces fuel efficiency and damages your engine components.

When your engine runs lean: it burns more oil. This is because exhaust gas has less oxygen than an engine needs to burn the same amount of fuel. When this happens over time, the increased pressure in your engine's cylinders can cause oil to leak into the cylinder walls and ignite. The resulting condition is called "Preignition" which can destroy an engine quickly. Having the right ratio of air to fuel is essential to getting the most power out of your car or truck. It’s also important for keeping your vehicle in good running condition and prolonging its life span.


Octane Level Imbalance

Gasoline can be mixed at higher or lower octane levels and this is known as mixed gas or gas blends. Gasoline with lower octane numbers, which is actually a less refined fuel, is typically mixed with premium fuel. High-tech engines have expensive computers and sensors that allow the engine to adjust itself to the mix of gasoline and automatically compensate for the difference in octane between regular and premium fuel. If you drive an older car that doesn't have this technology, mixing might not be such a good idea because you run the risk of damaging it. The biggest concern is the damaging potential of spark knock caused by volatile mixtures of ethanol and gasoline. Both have different octane ratings, which can be dangerous if you mix them. A spark knock occurs when the fuel-air mixture ignites prematurely before reaching its intended combustion point. This can cause permanent engine damage including worn valves, piston rings, and more.


Key Takeaways:

Gasoline contents are not designed to mix. All gasoline will deteriorate over time and mixing them will only cause more problems. Staying away from mixing can help you avoid costly repairs that may have otherwise been avoided.


Used the Wrong Gas on Your Vehicle? Seymour’s Garage Can Help!

Whether you need fuel pumps & lines replacement, fuel system injector cleaning, or an engine repair; we’ve got your back! Come see why we’ve been in business for over 50 years. Click or call today!


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