Nissan Repair: gas gustler, trip mileage, oxigen sensor

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 pump regulator?

Answer
what is your mpg?  do a fill up and reset your trip mileage to 0.  when empty calculate how many miles you went to the amount of gallons your car holds.  mine holds 15, im assuming yours is the same.  if its in the range of 22-26, you are in range it should be.  i have the same 2.4 4 cylinder and im getting 25 mpg.  if its a lot less than that...its not the o2 or else you would have a code for that.  fuel pump regulator...nah.  it only uses as much gas as it needs.  only other thing would be a leaky injector or a leak somewhere around the lines.  do the mileage test and get back to me and i will repost with another answer if none of these help.  xxp_sebby@yahoo.com  good luck!