When I had a car that could run flex fuel I test it out, running E85 for a few months, then regular for a few months. Turned out due to the decreased gas mileage, the price of E85 needed to be significantly less than regular in order for it to be cost effective. I can't remember the exact percentage right now, if I remember right, E85 was almost always that exact percentage cheaper, meaning it wouldn't save me any money at all, but I would be inconvenienced by having to fill up quite a bit more often.