From google bard:
"Ethanol has a higher octane number than gasoline, providing premium blending properties. Minimum octane number requirements for gasoline prevent
engine knocking and ensure drivability. Lower-octane gasoline is blended with 10% ethanol to attain the standard 87 octane.
Ethanol contains less energy per gallon than gasoline, to varying degrees, depending on the volume percentage of ethanol in the blend. Denatured
ethanol (98% ethanol) contains about 30% less energy than gasoline per gallon. Ethanol's impact on fuel economy is dependent on the ethanol content in
the fuel and whether an engine is optimized to run on gasoline or ethanol.
Ethanol also reduces harmful tailpipe emissions, such as carbon monoxide, exhaust hydrocarbons, air toxics, and fine particulate matter. A 2019 study
found that ethanol has the potential to emit 39-43% less harmful greenhouse gas emissions than gasoline.
Ethanol is made from corn, crop waste, wood chips, or sugarcane. Refineries add extra chemicals to make it poisonous and unfit for human consumption.
Most gasoline contains 10% ethanol, which is known as E10. The number after the "E" indicates the percentage of ethanol by volume in the fuel.
Ethanol-free gas contains about one third more energy than ethanol-based gas, so it can result in slightly better mileage.
Ethanol has about 67% of the energy content of gasoline per gallon, so using ethanol blends results in lower vehicle fuel economy (miles traveled per
gallon) than gasoline that does not contain ethanol. For example, vehicle fuel economy may decrease by about 3% when using E10."
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