I only went as far as E40 mixing E85 and E10 to see the effect on gas mileage as it was my engine and was designed to handle higher ethanol concentrations but not to E85. That was enough for me. E15 isn’t cheaper when factoring in fuel system damage and reduced gas mileage.
Well, or so maybe you thought. Fuel labeled as E85 can contain anywhere between 51% to 83% ethanol. So, you have some mix of ethanol there, but it probably wasn’t 40%.