Gasohol is a fuel blend made by mixing ethanol (alcohol derived from plant material) with conventional gasoline. In the United States, the most common form is E10, which contains 10% ethanol and 90% gasoline. Most gasoline sold at U.S. pumps already contains up to 10% ethanol, meaning the majority of American drivers have been using gasohol for years, whether they realized it or not.
Common Gasohol Blends
The two gasohol blends you’re most likely to encounter are E10 and E15. E10, the standard at most gas stations, is 10% ethanol and 90% gasoline. E15 contains between 10.5% and 15% ethanol. Both are classified as low-level blends, distinct from higher-ethanol fuels like E85, which is designed specifically for flex-fuel vehicles.
Ethanol is added for several reasons: it boosts octane, helps meet air quality standards, and satisfies federal requirements under the Renewable Fuel Standard, a policy that mandates a minimum volume of renewable fuel in the nation’s fuel supply. E15 has faced regulatory hurdles around year-round availability because ethanol increases evaporative emissions in warm weather. As of mid-2025, Congress and the EPA are still working toward a permanent solution to allow E15 sales nationwide during summer months.
How Gasohol Affects Your Engine
E10 gasohol has a pump octane range of 84 to 93, comparable to regular gasoline. That octane boost is one of ethanol’s primary selling points. However, ethanol contains less energy per gallon than gasoline, so blending it in reduces total energy content. E10 delivers roughly 112,000 to 116,000 BTUs per gallon, compared to about 120,000 BTUs for pure gasoline. In practical terms, that means E10 gives you about 3 to 4% fewer miles per gallon than ethanol-free gasoline. E15 drops fuel economy by roughly 5 to 7%.
For most modern cars, this tradeoff is built into the system. Manufacturers design engines to run on E10, and the slight mileage penalty is offset by the lower price of ethanol-blended fuel in many markets.
Potential Problems With Ethanol in Fuel
Ethanol is hydrophilic, meaning it actively absorbs moisture from the air. Over time, especially in fuel that sits unused for weeks or months, enough water can accumulate to trigger a process called phase separation. The ethanol and water bond together and sink to the bottom of the fuel tank, leaving a layer of lower-octane gasoline on top. The separated layer at the bottom can drop the remaining fuel’s octane rating to around 82 to 84, low enough to cause poor performance and engine-damaging pre-ignition (knocking).
If your engine draws fuel from the bottom of the tank, as most fuel-injected systems do, it pulls in the ethanol-water mixture instead of usable fuel. Water doesn’t combust well, leading to rough running, stalling, and accelerated corrosion of fuel system components. That water-ethanol mix also creates a hospitable environment for bacteria and fungi, which produce debris that clogs fuel filters and carburetors.
Ethanol is also a powerful solvent. It can dissolve deposits that have built up inside fuel lines and tanks, sending that loosened gunk through the engine. It dries out and degrades rubber seals, certain plastics, fiberglass, and even soft metals like aluminum and magnesium. It’s particularly corrosive to ferrous metals (anything containing iron, including steel), leaving behind salt deposits and a gel-like residue that further clogs the fuel system.
These problems are most severe in small engines like lawn mowers, chainsaws, motorcycles, and older vehicles. Unlike modern cars, this equipment typically lacks ethanol-resistant fuel lines, gaskets, and tank coatings. If you own small engine equipment or a classic vehicle, using ethanol-free gasoline when available can prevent these issues. At minimum, avoid letting gasohol sit in a tank for extended periods without a fuel stabilizer.
Environmental Benefits and Tradeoffs
The clearest environmental advantage of gasohol is cleaner tailpipe emissions. Ethanol contains oxygen, which promotes more complete combustion and reduces the formation of harmful byproducts. Research from Rochester Institute of Technology found that E20 (a 20% ethanol blend) reduced carbon monoxide emissions by an average of 23% and hydrocarbon emissions by 13% compared to conventional gasoline, with no measurable stress on the vehicles tested.
The carbon math is more complicated on the production side. Ethanol is typically made from corn in the U.S. through fermentation and distillation. Analysis from the National Renewable Energy Laboratory puts the net energy efficiency of a corn dry mill ethanol plant at about 54%, meaning that for every 100 units of energy in the corn feedstock, roughly 54 units come out as usable ethanol and electricity. Critics argue that when you factor in the energy needed to grow, harvest, and transport corn, the net climate benefit shrinks considerably. Supporters counter that cellulosic ethanol (made from agricultural waste like corn stalks rather than the grain itself) offers a much better energy return, though it remains a small fraction of production.
How Ethanol Fuel Is Produced
Most gasohol ethanol in the U.S. comes from corn. The process starts with milling the grain and converting its starches into simple sugars, then fermenting those sugars with yeast to produce alcohol. The resulting liquid is distilled to concentrate the ethanol, which is then dehydrated to remove residual water before blending with gasoline at fuel terminals. The blending happens close to the point of distribution rather than at refineries, because ethanol’s tendency to absorb water makes it impractical to ship through conventional petroleum pipelines.
Brazil, the other major gasohol market, produces its ethanol primarily from sugarcane, which yields more energy per acre and requires less fossil fuel input during processing. Brazilian gasohol typically contains 27% ethanol, a significantly higher blend than the U.S. standard.
What It Means at the Pump
If you drive a car made after 2001, E10 is safe and likely what you’ve been using all along. E15 is approved for vehicles from model year 2001 and newer. You’ll sometimes see E15 marketed as “Unleaded 88,” a reference to its octane rating, and it’s often a few cents cheaper per gallon than regular E10.
The fuel economy penalty is real but modest. On a 15-gallon tank, the 3 to 4% mileage drop from E10 works out to roughly 15 to 20 fewer miles per fill-up compared to ethanol-free gas. Whether that matters depends on the price difference in your area. In regions where ethanol-free gas costs 20 to 40 cents more per gallon, E10 is usually the better deal despite the lower mileage.
For seasonal equipment, boats, motorcycles, and anything that sits between uses, ethanol-free gasoline is worth the premium. Phase separation and the corrosion it triggers can cause repair bills that far exceed the savings from cheaper fuel.