I wasn't paying attention to what I typed, I saw the sig above my previous post and reading that I typed it in my response. Ment to say, if you have a "Tune" Depending on what it is, performance or otherwise, that may be your gas consumption problem. Sorry to confuse you OP if you read that.
I'm getting 20.2 in my 2010 GT as of today, I travel about 27 miles to and from work on roads that don't exceed 55. I run the "Performance" tune by Bama using 93 octane, prior to the tune I was getting 18-19 mpg.
I would hope you'd be getting ALOT better than 12.5, even if you stomp it all the time.