BIG problem ....!

I got a 2.7L 98 tacoma standard cab..... I had to recently change my o2 sensors. I turned the check engine light off and a couple of weeks later it came back on.. I got it diagnosed and it came back as "no problem found".... I got the light turned off and shortly afterwards it came back on.... still no problem found. I also noticed my gas is gettin sucked down! i calculated 14.6 mpg on my last 2 tanks. If it matters i run 32" all-terrains... Never had this problem before but i thought that maybe the low mpg and EC light might be related... I mean i knew my mpg would be low but never that low... any idea?
Also if that is normal does anyone have any ideas on tricks to increase my mpg....Every little bit counts