Engine Performance problem
2004 Nissan Xterra 6 cyl Four Wheel Drive Manual 37,800 miles
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I recently purchased a 2004 Xterra (5-speed 4X4, 3.3 L 6 cyl.) and the gas mileage is awful. Roughly 13 to 15 MPG combined. I traded in a 1998 Cherokee (5 speed 4X4 4.0 L 6 cyl) with 170K that I owned for 7 years that I got a standard 20 mpg from. Is there anything I can to do to try to improve mileage or diagnose a problem? Fuel injection cleaning, new fuel filter, O2 sensor, PVC etc? I'm not expecting a miracle but after all of the research I was expecting at least similar mileage to the jeep.

Thanks