Stop and go driving is not going to lend to great gas mileage. Known fact.
Look at my Caravan in my profile. Since the other half has been using it only in the city with NO highway driving, the mileage has dropped to half of what I was getting when driving it mostly highway. It went up some the last two tanks, due to a trip in the middle, but it still had a lot of city mixed in.
Personally I don't think you are having a mechanical or electrical problem, I think its reflecting your current usage. But that is my perspective.
Unless you had some proof (ie gas mileage logs) from your previous car on the same commute, that proved your last car did not grossly fail at achieving EPA ratings, taking it to the dealer is not going to get you anywhere. They're just going to give you the runaround and then try to get you to trade in for a hybrid at a huge loss. By driving poorly enough anybody can make their car perform that bad.
Where are the gas mileage logs from your 700 mile trip? What do those say?
My bet is poor/aggressive driving combined with extremely short trips and pure city environment.
Or rather, where are the mileage logs from the other 2000 miles on the car. You only have 710 miles logged but say there are 2700 on the odo. I sense the missing data may have some answers. Are you skipping fillups or did you just not start logging until there were 2000 miles on the odo?
Ugh, I know that but I've heard too many stories of people driving their brand new Kia off the lot and having something go wrong with the engine or anything and not being able to drive their car again for months... I'm glad I convinced my mother to get a Mazda 3 and not a Soul with the 1.6 engine.
2013 Mazda 3 GS-SKY 6MT (Current)
2015 Mazda 3 Sport GX 6MT (Lease return)
2013 Mazda 3 Sport SKY-SKY 6AT (Ex's daily driver - totalled)
2007 Toyota Matrix Base 5MT (Sold)