Is it Bad to Switch from Premium to Regular Gas?

With the rising cost of gas you may find yourself wondering if it’s ok to switch from premium to regular and vice versa. However, being a responsible automobile owner requires us to pay attention to our car’s fuel octane requirements and recommendations.

Modern vehicles have advanced engine control units that allow the engine to adapt to changes in octane. Switching between octanes should be done infrequently to avoid engine knocking.

The answer may be a little more complicated than you think, but in the end, there is always a clear decision that is best for you and your engine!

The Current State of Vehicle Gas Tolerances

For the majority of auto owners today, the fuel of choice at every gas station is regular, unleaded gasoline. This option is almost always indicated with an 87-octane rating at the pump. Most car owners return to pumps day in and day out with 87-octane gas in sight.

But once in a while, you come upon a filling station without regular unleaded gas. It can be distressing to encounter a situation like this, unless you’re informed on the current state of vehicles.

In the majority of cars produced within the last decade or two, they are technologically advanced enough to tell the difference between fuel octane types. That means, if you are forced to use premium gas (91, 92, or 93-octane), your engine isn’t going to struggle. 

Modern cars have electronic fuel management operations that will alter their mechanism whenever it recognizes a new fuel type in the tank. In most cases, the engine will adjust timing in the ignition and fuel injection. The result? A relatively smooth ride.

What’s the Difference Between Regular and Premium Gasoline?

I’ve made it clear that, in most cases, you will not do any long-term damage to your engine if you periodically or accidentally fill up your tank with the wrong fuel. But what exactly is the difference between the two? And what about diesel fuel?

When you park beside a fuel station, you will see a numerical tag beside each fuel type. This is the same country-wide. Regular gas has the tag of 87. Premium gas gets a tag of 91 up to 93.

The numbers designate differences in octane rating. The higher the octane rating, the higher the gas tolerance to compression before it fires or detonates.

In reality, the differences between 87-octane gasoline and 91-octane premium gasoline are minor.

Yet, over time, a car that requires premium gas will tend to underperform if you give it regular gas. Likewise, a regular gas car won’t fire correctly if you give it premium gas all the time.

The key to the equation is frequency. The difference between regular and premium gasoline will only become apparent in your car if you repeatedly serve your car the wrong type.

What Happens if You Switch from Premium to Regular Gas?

In the end, it’s not the best idea to switch from expensive fuels to cheap fuels for an extended period. However, the sophisticated ECU (Engine Control Unit) in most modern cars will tolerate a switch from premium to regular every once in a while.

But what happens when you switch octane types over a longer period? Not only will performance drop, but you’ll likely experience what is called an “engine knock.” 

An engine knock is an unwanted side-effect of giving a premium engine regular fuel. In the past, engine knock could damage the components inside your engine.

Today it’s not so serious. Engine knock occurs when the unburned part of gasoline self-combusts in a pattern that the engine cannot sustain.

Engine knock is the number one reason you should avoid fueling up your car with the unrecommended gasoline octane. It can disfigure an engine over time because the combustion of the gasoline occurs out of sync with the piston movements.

What to Look for When Determining the Right Gasoline

If you accidentally refill with an improper fuel type, the best response is to relax. As long as your automobile was made in the last ten years or so, the engine will tolerate it.

But auto owners should do their best to recognize which fuels to give their cars. What should you look out for when heading to the pump for your next refill?

Requiring Premium

Luckily, there is a hard-fast rule to help guide you. If your car requires premium gasoline – emphasis on “requires” – that means you need to purchase 91-octane fuel and above as often as possible.

A premium fuel requirement means your car needs higher octane fuel to achieve its intended height of performance.

Many European cars like BMW, Audi, and Volkswagen require their customers to purchase premium gasoline. This is because they mandate their cars to receive a higher level of detergent additives than the EPA requires.

You can check out the Top Tier Gasoline standards for more information as to why these producers have such high requirements.

Not Requiring Premium

If your car does not require premium – again, the keyword is “require” – then you do not need to buy 91-plus grade gasoline.

In the past, car owners would periodically flush out their car’s gas tank with premium gas. Why? 

Premium gasoline at one time offered an advantage over regular octane gas by including additives and detergents that improved performance.

Today, all octane levels of gasoline in the United States provide all the additives necessary to power any automobile, in line with the Federal Trade Commission

Why pay more for premium gasoline if regular gas is exactly what your car needs? Nowadays, there is no need to upgrade your fuel for added performance.

Recommending Premium

The last category of cars is those which recommend premium gas but do not necessarily require it. If you’re reading this article, there’s a good chance you fall in this hazy middle ground area. 

There is no one-size-fits-all solution for this group of cars and trucks. Unfortunately, many common autos fall in this category, including the Ford F-150 and Mazda Miata. 

The basic answer is, you won’t harm your car by purchasing regular gasoline. The manufacturer may claim that you increase performance with premium fuel, but regular gas will not harm your car.

Diesel Cars

With diesel autos, we have to become very serious with our recommendation. Do not fill a diesel car with regular or premium gas – or anything other than diesel, for that matter.

Doing so will cause irreparable harm to the internal parts. Why?

Unleaded fuel will strip away the lubrication inside your engine and cause massive damage after a while. 

Can you fill your regular-gasoline or premium-gasoline car with diesel? Good question.

It’s never a good idea!

Though filling a diesel car with regular is perhaps the worst situation, doing the opposite is second-worst. You’ll need to flush your fuel lines if you do so.

The Bottom Line

If in doubt about your car or truck’s fuel recommendation, there is a way to clear your suspicions up. Edmunds put together two lists for car owners to determine what fuel their cars truly need. 

If your car was made between 2012 and 2019, check out the list of Premium Recommended cars and Premium Required cars.

I hope this rundown of the basic rules behind fuel-octanes has given you a little more guidance about what to do at the gas station. At the end of the day, there’s no need to be overly worried if you accidentally switch from premium to regular gas.

However, a responsible auto owner should pay attention to their car’s needs and fill their engines accordingly.

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Hi, I'm the founder of VehicleAnswers.com! 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!