E85 is not regular gas. It’s a fuel blend made mostly of ethanol (alcohol derived from corn or other plants) mixed with gasoline. Where regular gas at the pump contains about 10% ethanol, E85 contains 51% to 83% ethanol, making it a fundamentally different fuel that only works in vehicles specifically built to handle it.
What’s Actually in E85
The name “E85” suggests 85% ethanol, but the real percentage shifts with the seasons. ASTM International, the organization that sets fuel standards, allows E85 to contain anywhere from 51% to 83% ethanol. In winter, the ethanol percentage drops closer to 51% because ethanol is harder to ignite in cold weather, and the extra gasoline helps engines start. In summer, the ethanol content climbs toward the higher end. The remaining portion is always conventional gasoline.
Regular gas (labeled E10 at most stations) is the opposite ratio: about 90% gasoline with just 10% ethanol blended in. That small amount of ethanol is standard in nearly all pump gas sold in the United States today. E85 flips that balance dramatically, and that difference changes how the fuel behaves in an engine.
Higher Octane, Lower Energy
One advantage E85 has over regular gas is a higher octane rating. Regular gasoline sits at 87 octane, while E85 typically lands between 100 and 105. Octane measures a fuel’s resistance to premature detonation (called engine knock), so E85 can handle higher compression without knocking. This is why E85 is popular in performance and racing applications where engines are tuned to exploit that knock resistance for more power.
The tradeoff is energy content. Ethanol packs fewer BTUs per gallon than gasoline. Pure ethanol contains about 76,330 BTUs per gallon compared to roughly 112,000 to 116,000 BTUs for standard gasoline. Because of this gap, one gallon of E85 holds only 73% to 83% of the energy in one gallon of regular gas. Your engine has to burn more E85 to produce the same amount of work, which directly affects how far you can drive on a tank.
Expect to Use More Fuel
The lower energy content translates to noticeably worse fuel economy. In real-world testing by Edmunds using a Chevrolet Tahoe, switching from regular gasoline to E85 dropped fuel economy by 20% to 26.5%, depending on driving conditions. That means a vehicle getting 21 miles per gallon on regular gas might get only 15 to 17 mpg on E85.
E85 is usually cheaper per gallon than regular gasoline, but the price difference rarely makes up for burning so much more of it. Whether E85 saves you money depends on the price spread in your area. If E85 costs 25% less per gallon, you roughly break even. If the discount is smaller than that, you’re spending more per mile.
Only Flex-Fuel Vehicles Can Use It
This is the most important practical detail: you cannot put E85 in a regular car. E85 requires a flex-fuel vehicle (FFV), which is engineered from the factory with fuel system components, sensors, and engine software designed to handle high ethanol concentrations. Flex-fuel vehicles have a yellow gas cap or a label near the fuel door identifying them as E85-compatible. If you’re unsure, your owner’s manual will confirm it.
Putting E85 in a non-flex-fuel vehicle causes a chain of problems. Ethanol has a different chemical ratio for combustion than gasoline, so the engine’s computer delivers the wrong fuel-to-air mixture. The result is hard starts (or failure to start at all), misfires, rough idling, and a check engine light. In cold weather, these problems get worse because ethanol is harder to vaporize.
Beyond the immediate performance issues, ethanol attracts moisture from the air. In vehicles without protected fuel systems, that moisture corrodes fuel pumps, injectors, and fuel lines over time. Rubber seals and gaskets not rated for high ethanol exposure can degrade, swell, or crack, eventually causing fuel leaks. A single accidental fill-up is unlikely to cause permanent damage if you dilute it with regular gas promptly, but repeated use will deteriorate components that weren’t designed for it.
Environmental Differences
E85 does offer a greenhouse gas advantage over regular gasoline. Corn-based ethanol, which makes up the vast majority of U.S. ethanol production, reduces lifecycle greenhouse gas emissions by about 40% compared to conventional gasoline. That figure accounts for everything from growing and processing the corn to burning the fuel in an engine. More advanced ethanol made from non-food plant materials (cellulosic ethanol) performs even better, cutting emissions by 88% to 108% compared to gasoline, though this type isn’t widely available yet.
The “lifecycle” distinction matters because ethanol still produces carbon dioxide when burned. The emissions benefit comes from the fact that the corn or plants absorbed CO2 while growing, partially offsetting what’s released at the tailpipe. Regular gasoline, made entirely from petroleum, has no such offset.
How to Tell Them Apart at the Pump
E85 pumps are clearly labeled and typically use a yellow nozzle or yellow handle to distinguish them from regular gasoline. Not every gas station carries E85. Availability is concentrated in the Midwest, where most U.S. ethanol is produced, though it has expanded to other regions. The U.S. Department of Energy maintains a station locator if you’re looking for E85 near you.
Regular gas (87 octane), mid-grade (89 octane), and premium (91 to 93 octane) all contain roughly 10% ethanol and are safe for any gasoline vehicle. E85 is in a separate category entirely, both in composition and in which vehicles can use it. If your car isn’t a flex-fuel vehicle, regular gas is the correct fuel, and E85 is not a substitute or upgrade.