87 octane gasoline in the U.S. typically contains about 10% ethanol by volume. This blend is commonly known as E10, where 10% of the fuel is ethanol and 90% is gasoline. The ethanol helps raise the octane rating of the fuel from the base gasoline level (around 84 octane) up to the standard 87 octane rating. Ethanol also acts as a cleaner-burning additive that reduces emissions compared to pure gasoline

. In summary:

• 87 octane gas usually contains 10% ethanol (E10).
• Ethanol boosts the octane rating and reduces harmful emissions.
• This is standard practice across most U.S. gasoline stations.

This 10% ethanol content is the norm unless otherwise specified as ethanol- free or a different blend like E15 or E85.