All you really need to know for the moment is that the universe is a lot more complicated than you might think, even if you start from a position of thinking it’s pretty damn complicated in the first place.
For several years now I have been driving a "Flex Fuel" vehicle - which means it can run on different blends of gasoline and ethanol. In my area I have a choice to use either Regular Unleaded or E-85 (85% ethanol fuel and 15% gasoline by volume.) The E-85 is typically priced substantially lower than Regular gas but there is a trade-off in that E-85 does not deliver the same mileage as Regular.
When fuel prices climb higher, the lower price of E-85 looks all the more attractive. However, it is hard to decide at the pump whether the lower cost is worth it, vis-à-vis the reduced fuel economy. In order to make an intelligent decision as to which would be more cost effective, I dusted off my limited javascript skills to devise this gizmo that painlessly compares the two options in terms of cost per mile.
Of course, it is up to you to discover your relative miles-per-gallon for each type of fuel. Once you are armed with that information you can enter those into the form below (my values are automatically inserted but you can change them to your own), along with the price of one or the other type fuel. Click the "EQUATE" button to learn the break-even price for the other.