(pbrady).This is one thread that has been covered over and over here under many different headings, altho not one quite so, 'to the point' as yours. The dealership who sold you your new car, hopes/prays you get good results but if you don't, no one really has a clue to what's going on unless your car throws a code for a fault somwhere in the air/fuel system. There has been some talk on the Versa Forum about bad O2 sensors and fuel regulators and you certainly do have a right to have that checked, as poor mpg's would certainly be a result of either condition.
There are also many of us who have had the complete diagnostics done because of the mpg issue, and the results were, no codes for anything out of order. So, that leaves us, basically nowhere from the dealers point of view, looking for a "fix" to the problem, so don't be suprised when they start lecturing you about bad driving habbits, poor fuel selection, your car isn't 'broken in' yet, etc etc.
I was driving a water truck for a living when I was 16..I am now 70, so I have been around the block a few times and get a chuckle out of all the advice about, how to; "drive like you have a raw egg under your foot, don't use the a/c, empty out the trunk, blow your tires up until they are like rocks, drive 10% under the speed limit" etc etc.
All good advice it you are in a hypermiling contest, but I like to carry a spare, a quiet safe ride at freeway speeds, usually have my wife of 50 years beside me and the dog sleeping in the back seat, and really like keep out of the way of that huge Truck looming in my rear view mirror.
For those owners who are indeed getting the EPA numbers or better, good for you, post those results on a thread that is about Great MPG's, for anyone who has found a solution to Horrible mpgs on THEIR Versa, please share that here, or here.
Good Luck trying to find the solution to your mpg problem, and I really mean that, for all of us in the same boat.