trying to figure out why I'm only getting 11mpg

Have a lift with p265 75 r16 tires and a truck box... Have been getting really bad mileage.. Check engine light is on p0420 bank 1... Put in a new upstream 02 sensor, code still returns... Added new oem spark plugs and an air filter.. Still not much better.. I don't haul anything or drive very fast... Could it be the front cat which I think needs replacing that's eating up gas or is it just the lift and stuff anyway?

 

Did you use the right O2 sensor? Did you reset the computer by unhooking the battery cables for 5-10 minutes?

 

Yes oem sensor.. New... Mebbe shouldn't have gotten it from ebay??? And reset light.. Comes back