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How does ethanol blended with gasoline affect octane rating and driving range?

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Labeled cross-sectional diagram of a V-type gasoline engine for an automobile, showing internal components such as pistons, crankshaft, camshaft, intake and exhaust valves, connecting rods, spark plugs, and intake manifold. — gasoline engine Cross section of a V-type engine.

Blending ethanol with gasoline significantly impacts both the fuel's performance characteristics and vehicle efficiency:

Octane Rating: Ethanol is an excellent motor fuel with a high octane number. Blending it with gasoline—typically in ratios such as 10% or 15%—raises the overall octane rating of the fuel, which helps prevent engine knocking and results in a smoother-running engine.12
Driving Range: While ethanol improves antiknock quality, it possesses a lower energy density than pure gasoline. Consequently, using an ethanol-blended fuel results in a decreased driving range per tankful compared to using unblended gasoline.1

While ethanol is a desirable, renewable alternative fuel, it is important to note that it should be used in fuel systems designed to withstand its chemical properties, specifically its tendency to dissolve certain plastic parts.32