Nissan Repair: why my car wastes so much gas?, fuel pressure regulator, oxygen sensors

Question
Hi, i got a 96 altima, and since i bought it i noticed it waste gas worst than a truck, i recently did a tune up and it cleared up the check engine light, but still waste gas like crazy, mechanic told me it must be the oxigen sensor but i doubt it, i was doing more research and im thinking it could be the fuel pressure regulator?, i had the car lift up to check for any leak but its all dry underneath i checked for leaks everywhere but nothing, one thing i noticed though i took one of the new spark plugs out and it was already black in the bottom, does this mean that the pump is delivering more fuel than it should? the car holds up to 15 gallons and did an mpg test it just gave me 272 mpg from a full tank,i think is ridiculous for a 4 cylinder engine, i really appreciate your help, thank you for taking the time to read this.

Answer
Charly,

Check the timing to make sure it is correct.  Change the oxygen sensor as it will kill mileage since old oxygen sensors get coated and lie to the computer telling it that the mixture is lean.  The result is the computer richens up the mix and your mileage goes to heck.  Also, check the fuel pressure to make sure you do not have leaky injectors or that the regulator has failed or the vacuum hose has come off of it.