I have an SEL w/ DCT - 16" wheels. My average since 6/8 is 35.2 mpg, which is city, highway and stop-and-go traffic. I gained .3 mpg since switching to full synthetic. Quality fuel seems to add in similar, almost insignificant numbers.
So, I would have to think it's your tire size and/or the resistance. If you're not throwing any codes and nothing is causing drag, anyway.