In the midwest, ethanol is often a key discussion point regarding alternative energy. I am not going to try to touch on the many political, environmental, and social issues regarding ethanol. Instead, I will touch base on a rather simply mathematical point.

It is often said that ethanol provides 70% of the energy of gasoline. Does this mean that your 30 mpg car will drop to 21 MPH (30mpg X 70%) when you use the ethanol pump?

No.

The 70% figure is the energy of straight ethanol (100% ethanol). What you see at gas stations is most commonly a 90/10 blend (90% gasoline, 10% ethanol) or E-85 (85% ethanol, 15% gasoline).

Let’s quickly run the math of the mileage for these two types of fuel.

  • For the 90/10 blend (by far the most common type of ethanol based fuel), 90% of the fuel (the gasoline) will provide 30 mpg while 10% of the fuel (the ethanol) will provide 21 mpg (30 X .70). When we use weighted averages (.9 X 30) + (.1 X 21) = 29.1 mpg. This is 97% of the fuel economy of gasoline.
  • For the E-85 blend (used in FlexFuel vehicles), 15% of the fuel (the gasoline) will provide 30 mpg while 85% of the fuel (the ethanol) will provide 21 mpg (30 X .70). When we use weighted averages (.15 X 30) + (.85 X 21) = 22.35 mpg. This is 74.5% of the fuel mileage of gasoline.

If you are trying to simply buy the most economical fuel, this should allow you to determine which fuel will provide the most miles per dollar.

Note: in some states, only 90/10 ethanol and E-85 is available – straight gasoline is not available. For this situation, bear in mind that E-85 provides 76.5% of the fuel mileage of the 90/10 blend.

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Kosmo is the founder of The Soap Boxers and writes on a variety of topics. Many of his short stories have been collected into Kindle books.

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