I've a new Accent which I'm very happy with except for the mileage. Most of my driving is rural (about 1 stopsign/light per mile), some highway, and very little city. I'm averaging 31.6. Hyundai claims 30 city/40 highway. I'll check the tires next. Your suggestions are appreciated.
It is okay to go over the tire pressure recommendations, but never go over the tire's max capacity written on the sidewall. If your car calls for 32psi and the tires are able to take 44psi try to stick around 35-36 and see if that changes anything. Your rolling resistance will be slightly better.
Another factor aside from highway speed is how smooth are your lane changes. You're still in the break-in period like overs have mentioned so don't worry about getting a 31.6mpg average to start with.
What were you driving before, what MPG's were you getting, and how did that compare to the EPA? Remember, the EPA number is based off a standardized test in a controlled environment (i.e. not driven in the real world). It's value is relative to other cars but not necessarily what you will see in use (hence the term, Your Mileage May Vary (YMMV)). Give the car some time to break in. Check your tire pressure to make sure they aren't low. You may consider having your alignment checked. But I'm guessing it is a combination of cold weather and lower tire pressures that will be the culprits. It will get better when the temps warm up in late spring.
When I first got my Smartcar I was a bit disappointed with my milage, I have since then put on 30,000 miles so far, My average is over 41 with the best being 46.5 mpg. So give your car a chance to get broke in and also give yourself a chance to know your car, good luck...