I have a 2011 GMC Sierra with a 5.3 and it's flex fuel capable. I burn both E85 and reg gas depending on the season. I've found that here in the Northeast (Long Island) E85 is more of a 3 season fuel,as it just doesn't run well when it's 15 degrees out. As I am typing this E85 by me is $3.19 vs $3.84 reg. When I gas up it's typically about a $15 to $20 dollar difference, depending on the price spread at the time of fill up. When I tow I notice only about a 1 to 3 MPG difference with the E85. My only gripe is that there aren't many stations that carry E85 in the Northeast. I also think I remember reading somewhere on the GMC website that horsepower is a little higher when using E85 (335 E85 vs 315 reg.)