E85 vs. 91 at 20psi – what are the main differences

For those who are not car enthusiasts, pounds per square inch denoted as PSI refers to the pressure contained in a vacuum system, such as engine compression levels or car tire inflation. You will find the recommended PSI range for your specific vehicle in your owner’s manual. Non-adherence to the recommended PSI range results in the poor performance of your vehicle, as your tires may wear out and you may experience reduced fuel efficiency. Now, let us look at what happens when you use e85 vs. 91 at 20psi.

E85 vs. 91

E85 gas, also known as flex-fuel, is an ethanol blend that consists of 85% ethanol and 15% gasoline. With this gas, only flex-fuel vehicles can safely use it and comfortably switch between refueling with regular gas and e85 gas. Modern cars, those manufactured after 2008, can also use ethanol blends, but they need a fuel system tune-up to protect its fuel system components and engine from corroding. Older vehicles, even when tuned up, experience difficulty when using e85 gas due to its corrosive nature.

E85 vs. 91

Higher octane rating

Octane ratings tell us the amount of air-fuel mixture that needs to compress before it spontaneously ignites. Gasoline with high octane ratings performs best when used in vehicles specifically designed to operate on that particular octane level. Lower octane gas, like that of regular gas, handles the least amount of compression before it ignites. The ratio of compression in your engine determines the rate of octane level your vehicle is most comfortable.

In the U.S., unleaded gasoline consists of different variations of octane rating; 87 (regular gas), 88-90 (midgrade gas), 91-94 (premium gas), and 93 (super premium). 91 gas, also referred to as premium gas, contains a higher octane rating than regular gasoline and e85 gas, meaning it burns more efficiently in the engine’s combustion chamber. Any vehicle model can use 91 gas. In most cases, using gas with a higher octane rating does not offer your vehicle any additional benefit.

high octane ratings

Is e85 gas cheaper than 91?

Generally, e85 gas is cheaper than regular and premium gasoline. Due to its easy process of manufacture and renewable nature, ethanol blend fuel stations are becoming more popular all over the U.S. However; cheaper does not always mean affordable. E85 gas has lower energy content, meaning you may end up paying more in the long run.

According to mechanics at Edmunds, e85 gas completes fewer miles per gallon as compared to a vehicle that uses regular gas. As a result, you may find yourself spending more money on refilling e85 gas; hence, more expensive in the long run. However, renewable gas burns cleaner than 91; thus, it is more eco-friendly since it has fewer toxic gas emissions.

Premium gas is usually the most expensive gasoline you can buy at a gas station, costing about 50 cents per gallon. Despite its higher price tag, it does not promise better engine performance, fuel economy, or better emissions control. The gas is expensive, mainly because of the additives in it, which cost a lot in production. Many professionals agree that buying more octane than your vehicle requires is a complete waste of resources.

Lower exhaust gas temperatures

Air fuel ratio, spark timing, and boost level actively contributes to the rate of exhaust gas temperature. E85 gas provides lower exhaust gas temperatures as compared to 91 gas, due to their evaporative cooling effect. Gasoline is known to atomize when introduced into the engine while ethanol turns to vapor.

Liquids tend to absorb immense heat when they vaporize, adding weight, and increasing complexity and cost of engine maintenance. As such, 91 gas reduces the efficiency of the engine’s cooling system while E85 gas knocks down inlet air temperatures, leaving no need for the intercooler.

Heat is an enemy of a vehicle’s engine as it may cause it to stop working due to overheating. Lower exhaust gas temperatures show the temperatures in the engine are favorable for its optimal performance.


With more than 22 million FFVs in the world, e85 gas is quickly gaining popularity. Furthermore, federal governments have been pushing us towards using more ethanol blends in our vehicles as opposed to regular gasoline. By shifting to alternative sources of fuel, the world’s oil supply will increase due to their renewable nature.

Premium gas (91-93) is best for vehicles with high compression engines. However, due to the availability of knock sensors and computers that adjust your vehicle’s settings to use various fuel grades optimally, modern cars can use regular gas or e85 gas without voiding their new car warranty. When choosing between e85 vs. 91 at 20psi, consider why you should pay your hard-earned money for something you do not necessarily need.

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