You'd have to re-tune your engine for e85, e85's stoich is way richer than regular unleaded's 14.64. e85 also requires a higher flow of fuel, so in most cases you need bigger injectors and sometimes a different fuel pump.

 

I have always owned a Mustang since I have been driving, 2 v6's before the GT. I work hard to have it but I'm not just gonna drive horrible and get 15mpg with gas what it is right now since it is my only car and I go 20 miles for work.