The payback is not the difference in price. Ethanol has less energy per gallon which results in less miles per gallon. It ranges from about 25 to 40 percent based on the ethanol content. That translates to 20 to 40 percent more gallons needed to go the same number of miles as gasoline. In addition, the example you used assumes the price gap will remain constant. There may still be a payback but not as much as the example you used. However calculated it will fluctuate as the supply and demand changes.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas