The Washington Post recently ran my letter to the editor to correct an error found in a review of the Chrysler 300. While the letter was just a technical correction, here’s my full take on ethanol’s impact on miles per gallon.
The column stated that the V6 300’s fuel economy is 19 mpg in the city and 31 mpg on the highway (these are EPA test results) “using regular or E85 (85 percent ethanol) gasoline.”
This is thermodynamically impossible. (I have a chemical engineering degree. I’m required to refer to thermodynamics whenever possible.) The energy content of ethanol is about 67 percent of that of ethanol-free gasoline so you can’t expect an engine to push a car as far using an ethanol blend. As ethanol is blended with hydrocarbons the average energy content of the gasoline is diluted. Below are the relative reductions (theoretically, your mileage may vary):
• Gasoline with no ethanol (E0) 0% (i.e. non-ethanol gasoline is the standard)
• Gasoline with 10% ethanol (E10) 3-4%
• Gasoline with 51% ethanol (E85) 17% (E85 must contain a minimum of 51% ethanol)
• Gasoline with 83% ethanol (E85) 27% (E85 may contain up to 83% ethanol)
If you are thinking about purchasing a vehicle you should keep in mind that the EPA fuel economy figures reported on the vehicle sticker are based on a test that uses a non-ethanol gasoline. Since almost all gasoline in the United States now contains 10 percent ethanol you should expect your new vehicle’s fuel economy to be 3-4 percent less than reported. If you are considering the purchase of a flex fuel vehicle (FFV) that can operate on E85 then you can find the FFV fuel economy test results on the EPA web site. For the Chrysler 300 the fuel economy values reported for E85 are 14 mpg in the city and 23 mpg on the highway which is only 74 percent of the fuel economy reported for the non-ethanol test fuel.
Once you own an FFV and you have to make a decision to fill up with E10 or E85 you just need to keep in mind that the energy content breakeven point is at about 75 percent, i.e. if a gallon of E85 costs less than 75 percent of the E10 price then you should fill up on E85; if a gallon of E85 costs more than 75 percent of the E10 price then you should fill up on E10.
And don’t forget – your mileage may vary.