Better tire wear and better gas mileage.

NEVER use the pressure rating on the tire. That is only the recommended MAX pressure that tire can take. The only tire pressure you should follow is located on the placard either on the inside of the door under the latch, or on the door jam under the latch receiver. Using the tire pressure on the tire can result in poor performance, traction, tire blowouts, uneven tire wear, and poor gas mileage. The manufacturer factors in details the tire could never know, such as weight of the vehicle, alignment specifications, and performance of the vehicle.

Good gas mileage on this car should be 19City 27Hwy. Gas mileage can be affected by many things. Assuming that the car has been recently tuned as you have indicated, I would: Check tire pressure first<-you'd be surprised what low tire pressure can do. Check for low oil level <-engine friction Cheap spark plugs. Let me know.

If you run low tire pressure you can get cupping on the sides of the tires. Tire pressure needs to be checked regularly and kept at the recommended pressure or above.
Another cause of tire cupping can be bad or loose steering head bearings. Loose bearings will cause the wheel to wobble when decelerating just like low tire pressure. It can sometimes be detected just by turning the handle bar from side to side. If it feels like the steering wants to lock in the driving straight position or you feel ripples or bumps while you turn the bar from side to side you need new steering head bearings and races.

Driving habits...incorrect tire pressure...excessive weight in car...low oil/old oil. Many things affect mileage. My 2002 max needed a new O2 sensor. Once replaced the mileage improved slightly.

These are the most common causes for bad mileage.
Tire pressure is a huge factor (Tire should be set at the factory recommendation or 1-2 psi above, but never below - Tire should also be a Low Rolling Resistance Tire)
Oil level (you said it's OK, so I assume it's not above the full mark)
Running extra accessories (A/C, etc)
Dirty Air Filter
bad Ignition timing
engine running hotter than normal
fuel injectors may be stuck partially open
bad fuel pressure regulator
faulty emission control device (this usually triggers a check engine light)