I would definately get it checked out. I get between 21-24 mpg with about 55% city and 45% highway driving. When 100% highway I get about 26-30 depending on what season. I have a 2003 manual.
Things to check yourself though:
1. tire pressure(make sure its not too low)
2. air filter(make sure its white or light gray if using stock and not pitch black or really dark gray)
3. take a quick look at the tires on the front and back, look for one side different than the other(leaning in or out), note: the rear tires are leaning in and its on purpose so dont worry about that.
4. drive with a light foot, if you gradually accelerate rather than blast off hammering the gas, you will save at least 10%.
5. make sure you dont overload it. If you drive with full passengers or load you will get less gas mileage, its only a V4 afterall.
All else fails, take it in and have someone look at it. It should still be under warranty unless you are over mileage.
Just my thoughts