This thread makes my head hurt. I don't think E15 really makes that much of a difference. 1.) Ethanol isn't really that bad for modern fuel systems. The problem is that it's hydroscopic, so it attracts water to the fuel system, creating rust over time. If you have a modern fuel system then you really don't have to worry about the rust issue presenting itself there. I run E15 in Michigan 6 months out of the year and have seen zero difference in my polymer tank's characteristics. E15 is not going to hurt your engine. 2.) Yes, ethanol has a lower energy content, but it also has a larger octane rating. I'm unsure of how large it is when it's in E15 though. In E85, the octane rating is way higher so it's great for boosted applications and highly advanced timing. The one catch being that you need to put more fuel in like Brian stated so you're seeing a richer A/F. I'll be able to tell you how much my blown LS1 loves the corn juice soon hopefully. My fuel mileage drops around 1 mpg to 2 mpg when I run solely E15 for extended periods of time in both my bike and car. Everyone thinking that E15 is the devil, um ok...