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E-85 Flex Fuel vs. 91-93 Octane (Differences & Similarities)

E-85 Flex Fuel vs. 91-93 Octane (Differences & Similarities)

We’ve written extensively about different fuels and what cars should and shouldn’t use them. However, one question we haven’t answered is what the actual differences and similarities are between E-85 and premium gasoline with an octane of 91 – 93. Here’s a quick answer:

The difference between E-85 and premium gasoline with an octane of 91 – 93 is that E-85, on average, produces 20% more horsepower, is 33% cheaper, and produces 40% fewer emissions. However, E-85 also has a 20 – 30% lower MPG and increases the amount of maintenance of the fuel system.

However, that certainly doesn’t answer the question wholly. Below, we’ll first start with a quick introduction to both types of fuels and their characteristics. Furthermore, we’ll discuss the six differences between these types of gasoline. We’ll also discuss what cars can use what type of gasoline and if it’s a good idea to mix them. Read on!

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Want to know more about the difference between E-85 and regular gasoline with an octane of 87? Read this blog: E-85 Flex Fuel vs. 87 Octane (Differences & Similarities)

A Quick Introduction To E-85 and 91-93 Octane Gasoline

Before we dive deeper into the topic, it’s essential to know that E-85 and 91-93 octane gasoline is made out of different fluids. This means both types of gasoline can’t be used interchangeably, and you should be careful with exchanging one for the other.

E-85 stands for ‘ethanol 85%’, which means the gas you use is made out of 85% ethanol and 15% regular unleaded gasoline with an octane rating of 87 – 93. However, the E-85 for the United States is slightly different. This is because, confusingly enough, E-85 only contains 51 – 83% ethanol in the United States, whereas the rest of the world generally goes with 85%.

Finally, ethanol is made out of different materials, but all of them are considered environmentally friendly. These materials included corn and bio-waste.

Also read: 7 Differences Between E-85 And Bio-Diesel

On the other hand, there’s 91-93 octane gasoline which we refer to as premium gasoline. This is 85 – 90% regular unleaded gasoline with an octane rating of 91-93 and 10 – 15% ethanol. It is considered less environmentally friendly because its main ingredient is crude oil. However, it is what most cars use.

The difference between 91 and 93 octane itself is negligible and only has to do with what type of engine the car manufacturer uses and what fuel they advise. Some high-performance vehicles preferred 93 octane instead of 91, which will be written down in the owner’s manual.

Comparing E-85 to 91-93 Octane

Both types of gasoline come with distinct advantages and disadvantages. Below, we’ve listed six factors that will show us the inherent differences between these fuel types.

Also read: The Best 93 Octane Gas Is Sold By These Retailers

Octane Rating

One of the main differences between these fuel types is the octane rating itself. As you’ll be able to understand, 91-93 octane has an octane rating of 91-93. However, E-85 typically has an octane rating of 100 – 113 (depending on the seasonal blend used). However, remember it has a lower octane rating in the United States because they use a lower percentage of ethanol.

A higher octane rating means the fuel is more stable and can handle a higher compression ratio—a higher compression ratio results in more mechanical power, which means a higher power output and more speed. The high octane rating of E-85 is one of the reasons why E-85 is used in many race cars.

Also read: How Much Ethanol In 87 – 93 Octane Gas? (Regular & Premium)


We know now that E-85 (with 85% ethanol) generates more horsepower than 93 octane gasoline in engines made to handle these types of fuel. However, how much horsepower are we talking about exactly?

One example is the Koenigsegg CCXR which has a 20% higher power output when using E-85 compared to 93 octane gasoline. Furthermore, this company did testing with their 2016 Twin Turbo V6 Dodge Challenger and found that it produced 64 more hp and 38 more torque with E-85 compared to 93 octane.

Again, this shows us that the higher octane rating of E-85 results in significantly higher power output for high-performance cars that can handle this type of fuel. In Australia, where E-85 indeed contains 85% ethanol, it’s therefore used regularly in racing cars. In the United States, this is not the case because of the lower ethanol and, therefore, lower octane.

Fuel Economy

The lower burn rate combined with, the higher power output means E-85 bio-fuel is a lot less economical than regular unleaded gasoline. Research has shown that E-85 makes the MPG drop by 20 – 30%, depending on your car.

However, people in favor of biofuels (which is what E-85 is) have stated many times that there are engines that have shown to be 20% more effective running on E-85 than other gasoline. They blame car makers for not designing the right engine and modifying gasoline engines to make them work with E-85, whereas a modified diesel engine would work much better.


However, the drop in fuel economy is made up for because E-85 is typically a dollar and a half per gallon cheaper than 93 octane gasoline. At the time of writing, 93 octane gasoline has a national average price of $4.60 per gallon, whereas E-85 is $3.10.

In conclusion, this means E-85 is about 33% cheaper than premium octane gasoline and 20 – 30% less effective in fuel economy. This means E-85 is slightly more affordable in the long run when we compare these two factors.

Also read: Is E-85 Cheaper To Use Than Other Gasoline? (Explained)

Maintenance And Lifespan

One other major thing to consider when comparing E-85 to premium gasoline is the car’s maintenance. There’s no denying that cars that run on E-85 need more maintenance and more frequent checkups of certain vehicle parts. These include the spark plugs, valves, and fuel lines of the fuel system.

This is because these elements have a much higher wear-and-tear when using E-85. If you run your car on E-85 or are planning on doing so, we highly advise you to check your car’s maintenance schedule. If your car is made for use with E-85, then the carmaker will have published a maintenance schedule that’s specific to E-85 vehicles.

Emissions & Enviromental Impact

The final factor we need to discuss is the emissions created by the different types of gasoline. Many people assume that E-85 vehicles are cleaner because they burn bio-fuel. However, studies around this subject are inconclusive. This is mainly because of the vast number of variables in play during the research (the type of car, engine, percentage of ethanol, conditions, etc.). Because of this, we can’t make hard conclusions about this.

However, E-85 emissions are considerably lower when we compare them to premium gasoline (or any gasoline) on a life-cycle basis. This is because E-85 is much more environmentally friendly to produce, and the crops that are grown also take in carbon dioxide when they’re still on the field. According to the US Department of Energy, emissions of E-85 that are based on corn are 40% lower than the emissions of regular or premium gasoline, making E-85 the superior choice.

Can My Car Use E-85 and 91-93 Octane?

Whether or not your car can run on E-85 or 93 octane, and which one would be the best option, depends on several factors.

First, you’ll have to know what car you have and what gasoline the carmaker prefers. This information is mentioned in the owner’s manual. On the other hand, you can also search on this website by typing in ‘car model + gas’ in the search bar. We’ve written an article for almost every type of car sold in the United States.

E-85 can only be used in so-called ‘Flex Fuel’ cars. Flex Fuel cars are cars that have a fuel system designed to use E-85 gasoline. In general, these cars will have badges or markings on the car, which notifies you of the potential use of E-85. See the image below.

Most of the time E-85 is used in cars that have a large engine with a lot of horsepowers.

As a general rule of thumb, we would say that all gasoline-powered cars can make use of 93 octane gasoline. This is because this type of gasoline consists of 85 – 90% gasoline and 10 – 15% ethanol. That’s the exact mixture that the large majority of cars run on worldwide.

However, using 93 octane is not always recommended. As we saw earlier, 93 octane is considered premium gasoline, which costs more. Furthermore, most cars will already reach their optimal fuel efficiency and power output using 87, 89, or 91 octane gasoline.

93 octane gasoline is only recommended if the owner’s manual of your car states that 93 octane gasoline should be used. This is usually the case for luxury or power-focussed car models with large engines.

Using 93 octane in an engine that’s designed for 87 – 91 octane gasoline will only result in more cost and won’t have any specific performance benefits.

Can You Mix E-85 and 91-93 Octane?

Theoretically, it’s possible to mix E-85 with premium gasoline. However, this is undoubtedly not recommended for the average consumer.

First, let it be clear that this can only be done when your car is already approved for E-85 gasoline. If you mix E-85 with premium gasoline in a car that’s not supposed to run on more than 10 – 15% ethanol, you’ll most likely end up with gasoline that far exceeds that number. This will permanently damage your engine in a way that’s not covered by any form of warranty.

Assuming you have a car that can run on E-85, it all comes down to simple mathematics. If you mix 5 gallons of E-85 with an octane rating of 105 and 5 gallons of gasoline with an octane rating of 93, you’ll end up with 10 gallons of 99 octane gasoline.

Most people who decide to mix these forms of gasoline are looking to spend a little bit of money and achieve a little higher octane rating. However, we would advise not taking this amount of risk with your fuel system. If anything goes wrong, you’ll end up with a car that has a broken down fuel system and no warranty to cover it. Instead, buy the gasoline with the proper octane number for your car.

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