really common to see with leaky headers and closed loop. The o2 sensor picks up fresh air readings, thinks it is driving around death valley and its -30 degrees... then and adds a crap load of fuel when its not needed.

If not leaky headers then it would have plenty to do with the o2 sensor and or some other common issues. My gtp has never gotten less than 24mpg, and my crappy GTP with plug wires/plugs so bad it wont really even run at wot, and has a broken o2 sensor, gets 19-20.