Welcome to the Forum Mario.
Have you checked the air filter? Is the tire pressure what it should be (35 psi)? The Sport came with 255/50R20 tires. Is that what is on there now? What octane are you using? Premium will increase it slightly but whether it is worth the extra cost is debatable. If you're going by the MPG readout on the dash, then I've got bad news for you. The readout is usually a bit on the generous side by about 1 MPG or so. You'd be better off using pen and paper or a site like Fuelly.com which I have been using for my past 3 vehicles. It will do all the figuring for you after you punch in the numbers. The Manual suggests changing plugs at 100k miles but it may not hurt to check a couple of them.
Peter