My 2011 XL Denali just started spitting crazy fuel MPG. Starts in V8 with regular mpg but when I slow down, it turns on V4 and the mpg goes way up when stopping. The only thing different recently is using E85 to top off my tank. This morning, it was really sluggish and hard to excel between 30-55. Tonight it was fine but still had the weird mpg stuff Anyy ideas? Range AFM device will be here Saturday, thankfully.