I've been a long time lurker here, but I just want to post my fuel mileage situation.
Vehicle: 2014 Silverado LT 5.3, 3.08 gear ratio, 2.5" level, 32" tires.
Mileage: I bought the truck at ~30k miles, I was getting an average of 16.5 mpg up in Alaska on winter blend fuel, moved down to FL and now I'm getting at best 13.5 mpg with mid grade and as low as 11.5 with E85. I do 50 miles highway and about 3 miles city every day and I drive very conservatively.
I didn't buy the truck for its fuel economy, but something seems off.