If I remember correctly, the E85 gasoline/ethanol blend is usually cheaper, but the gas mileage is lower, so whatever cost savings on the fill up you realize is spent on reduced fuel economy. I would check with a Mustang forum or a local Ford dealer and see what they have to say. The selling points of using E85 are helping with smog emissions, subsidizing farmers with their corn crops, and using less oil to make a gallon of gas.