Sudden drop in mpg

A couple of months ago I had the catalytic converter and oxygen sensor replaced under warranty on my 2005 Tacoma TRD. A little over a month ago my gas mileage started dropping and has continued to do so. I used to average 17 mpg. My driving and other conditions have remained the same, but my last fills have gone from 17 to 15.4 to 14.8 to 14 and the last one was under 12. (I typically fill when the tank gets down to half.)

Could it be a bad oxygen sensor? Has anyone else experienced anything like this?

 

It could be the O2 sensor. One of the most overlooked causes of bad mileage though is the MAF and throttle body. Both need to be cleaned regularly. I had a Suzuki that started running terrible fairly suddenly and it turned out to be the TB. I'm assuming plugs and air filter were changed on time?