Rajan Wilcox
Flex-fuel vehicles are capable of running on both ethanol-gasoline blends and regular gasoline. E85, a blend of 51% to 83% ethanol and gasoline, is the most common form of flex fuel available on the market. While E85 gas costs less than regular gasoline at the pump, the decrease in miles per gallon makes it more expensive when driving. However, flex-fuel vehicles often put out more torque and horsepower when running on E85. Octane is the fuel's resistance to detonation, and E85 has an octane rating above 93. Therefore, it is suitable for use in any vehicle designated as a flex-fuel vehicle by the manufacturer.
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What You'll Learn
E85 gas is a type of flex fuel
Flex fuel is an alternative fuel blend consisting of around 85% ethanol and 15% gasoline. It is also known as E85 and can only be used in flex-fuel vehicles, which are specially designed to run on this blend. E85 is not compatible with traditional gasoline engines and will damage them.
E85 is a domestically produced, renewable biofuel that has been available since the 1990s. It is made from sustainable crops, primarily corn, grown in the United States. This type of flex fuel burns cleaner than pure gasoline, resulting in reduced greenhouse gas emissions for vehicles. Additionally, E85 typically has a higher octane rating, which can improve performance in tuner cars.
One of the considerations when using E85 is its impact on fuel economy. While E85 typically costs less per gallon than regular unleaded gasoline, vehicles using it travel fewer miles per gallon. This lower fuel efficiency is a trade-off for the environmental and performance benefits offered by E85.
To identify a flex-fuel vehicle, look for exterior badging or a yellow fuel cap, indicating compatibility with E85. Additionally, the owner's manual or manufacturer's website should provide information on the vehicle's preferred fuel type. As of March 2025, there were over 4,200 public gas stations in 44 US states selling E85, with a higher concentration in the Midwest, a major corn-producing region.
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Pros and cons of E85 gas
E85 is an alternative fuel that is also known as flex fuel. It is a mixture of ethanol and gasoline, typically with 85% ethanol and 15% gasoline, but this can vary. For example, some variants have 50% ethanol and 50% gasoline. The mixture usually has more ethanol during the summer compared to the winter due to ethanol's different evaporation characteristics.
Pros of E85 Gas
- It is cheaper per gallon than premium gasoline.
- It has a smaller carbon footprint and burns cleaner than traditional gasoline.
- It is a renewable alternative to fossil fuels, which will eventually run out.
- Some GM FlexFuel engines can produce more horsepower when run on high-octane E85.
- It has an octane rating above 93, which is why it is used for higher performance in tuner cars.
Cons of E85 Gas
- E85 has poorer fuel economy than gasoline, with users reporting an MPG decrease of between 18% and 25%.
- E85 doesn't store as well as gasoline as ethanol absorbs moisture.
- E85 has inconsistent ethanol content and octane rating at the pump.
- Flex-fuel vehicles have more expensive parts and repairs than traditional gasoline vehicles.
- Ethanol is naturally corrosive and can cause storage problems, water issues, and engine damage over time.
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Octane is the fuel's resistance to detonation
The octane rating of gasoline is measured in a test engine and is defined by comparison with the mixture of 2,2,4-trimethylpentane (iso-octane) and normal heptane, which would have the same anti-knocking capability as the fuel under test. The percentage, by volume, of 2,2,4-trimethylpentane in that mixture is the octane number of the fuel. For example, gasoline with the same knocking characteristics as a mixture of 90% iso-octane and 10% heptane would have an octane rating of 90.
The higher the octane of the fuel, the harder that fuel is to ignite, and the more heat is required to ignite it. The result is that a hotter ignition spark is required for ignition. Creating a hotter spark requires more energy from the ignition system, which in turn increases the parasitic electrical load on the engine.
Ideally, the air/fuel mixture will ignite at the proper time and burn smoothly through the power stroke. The idea is that one powerful combustion of the air/fuel mixture is better than several randomly ignited small flame fronts. When you can precisely control the point at which the fuel will ignite, maximum performance of the engine can be achieved, and power-robbing knock and ping will be eliminated. Knock and ping are a result of abnormal ignition, or multiple flame fronts colliding within the combustion chamber during the compression stroke.
Detonation can happen from hotter-than-normal temperatures from carbon in the combustion chamber, which higher octane helps to prevent.
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Premium fuel in a flex fuel vehicle
Flex fuel is not a type of fuel but a type of engine that can take ethanol-enriched fuels as well as regular fuel. E85, a blend of ethanol and gasoline, is the most common form of flex fuel available on the market. It typically has an octane rating above 93. Other types of fuel that can be used in a flex-fuel vehicle include E15, regular 87, 89, and 93.
There is no benefit to putting premium fuel in an engine that doesn't require it. However, if a flex-fuel vehicle is not designed to take premium fuel, there is also no benefit to using it. E85 burns cleaner than pure gasoline, but it is less efficient, with lower gas mileage. It is also more expensive when driving due to the decrease in miles per gallon, despite being cheaper at the pump.
If you are mixing fuel types, you can use an Octane Mix Calculator to determine the right ratio. For example, to get 93 octane, you would need to mix 12 gallons of 88 octane and 8 gallons of E85.
Some people prefer to use premium fuel in their premium vehicles, and some vehicle types will always benefit from higher octane fuel. For example, any turbo vehicle will benefit from a higher octane fuel compared to an NA motor.
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Calculating the right mix of E85 and 93 octane
Flex fuel vehicles can typically take ethanol-enriched fuels as well as regular fuel. This includes E85, which has an octane rating of 104 or 105. Mixing E85 with 93 octane fuel can produce 100 octane fuel. However, it is important to note that E85 has less thermal efficiency than gas, and the two fuels have different weights and burn rates. Therefore, simply mixing them to max out the map slot is not advised.
If you are running a 93 octane map and want to be extra safe, you can add a few gallons of E85 to keep the octane up and knock down. However, trying to mix the right amount of E85 with 93 octane fuel to equal 100 octane is not recommended. The two fuels have different burn rates, and mixing them can affect the performance of the vehicle.
To calculate the right mix of E85 and 93 octane fuel, you need to consider the desired octane rating and the volume of fuel. A tuner can help determine the correct mix, as the car's ECU needs to be "told" to push more volume with each injector pulse when switching to E85. Additionally, the injectors need to be able to handle a large increase in fuel volume, usually twice the CC's currently in the vehicle.
One calculator suggests that 11 gallons of E85 and 8 gallons of 93 octane fuel will give a 100 octane formula. Another calculator that specifically accounts for E85 and 93 octane fuel, instead of just octane ratings, resulted in a 50/50 mix, which is essentially E50. However, a 50/50 mix may not work due to the difference in burn rates between the two fuels. A tuner recommended a maximum of 2 gallons of E85 in this mix.
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Frequently asked questions
Yes, you can use 93 octane in a flex fuel car. Flex fuel cars can use ethanol-enriched fuels as well as regular fuel.
Flex fuel is not a type of fuel, but a term used to describe a car that can use ethanol-enriched fuels as well as regular fuel.
E85 is a type of flex fuel that is a combination of ethanol and gasoline, consisting of 51% to 83% ethanol, with the percentage of ethanol depending on the location and time of year.
Yes, you can mix E85 with 93 octane, but it is recommended to use straight 93 octane as your motor will not benefit from much more than 93 octane.
There may be no benefit to using premium fuel in a flex fuel car if the vehicle is not designed to take premium fuel. However, some argue that premium fuel can provide more engine power and increase miles per gallon.
