That is low, something is wrong. I get about 18 city/28 highway, up to 34 on road trips. It could be the OBC, and being a Base or S and driving style plays into it, but not that much.
My first thought would be to keep it simple and disconnect the battery then reconnect it. That will reset all of the computers, and may help the car recalibrate itself to be more efficient. My next move would be to clean the throttle body.
http://www.pedrosgarage.com/Site_3/Clean_the_Throttle_Body.html