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Understanding the Difference Between Premium Gasoline and Regular Gasoline

There is a pervasive idea that using premium grade gas is better for your car than regular gasoline. Some people claim that it makes your car run more efficiently and will improve fuel economy.  Also, producers mix in special additives like detergents that keep the fuel system clean. It makes sense that using better fuel would increase performance, right?  In reality, most cars don’t require premium level gasoline and would actually be better served by using regular gas. To understand why, we first need to break down how gasoline is rated.

The Meaning of Octane Rating Numbers

A major component of gasoline is a group of flammable chemical compounds called octanes. Researchers use a specific octane, isooctane, as the standard for measuring how much compression the gas can withstand before combusting. If the fuel ignites prematurely while being compressed in the cylinder, it can create small explosions inside the engine. Known as knocking, these explosions will decrease your car’s performance and can seriously damage the engine. 

The numbers you see on fuel pumps show how the gas performed against 100% isooctane. In the US, gas stations typically offer octane ratings of 87 (regular), 88–90 (mid-grade), and 91–94 (premium). Higher octane ratings mean the fuel can withstand more compression before igniting, which decreases the potential for knocking. Lower octane ratings mean the fuel will ignite more easily when compressed, which increases the chance of your engine knocking. 

Premium Gasoline and High-Pressure Engines

Another part of the premium versus regular gasoline question is whether or not your car has a high-compression engine, is equipped with supercharging, or has turbocharging. These systems work by drawing more air into the cylinder to mix with the fuel and increase the amount of compression on the fuel mixture. The higher compression requires premium gas to prevent engine knocking. However, most vehicle engines don’t produce enough compression to take advantage of the higher octane levels of premium gas. Instead, check your car’s owner manual to find the recommended fuel rating to use. 

What Happens If I Use The Wrong Gas?

In the event you fill up with premium gasoline when your engine only needs regular or mid-grade, it’s unlikely to cause you any major trouble beyond a possible drop in fuel economy. Just go back to the recommended fuel grade the next time you fill up. To get the same “cleaning” benefits of premium gas, you can easily buy a bottle of fuel system cleaner to add to your tank. 

Using regular gas in an engine calibrated for premium is more problematic due to the increased likelihood of knocking.  While many newer vehicles can adjust the spark timing to help reduce the severity of the knocking, your engine will still be running poorly. Prolonged use of the wrong fuel grade can irreversibly damage the engine and emissions control system. You should switch back to premium gasoline as soon as possible as well as visit an auto repair shop to have them run a diagnostic scan and check for any damage.

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