There are tons of things that can affect MPG. The state of maintenance on the engine in general, spark plugs, O2 sensors, catalytic converters, MAF sensor, clogged intake or throttle body. What possibilities have you eliminated?
Dang, I had it backwards and said downstream. I over thought it.Change the O2 sensors also, use only AC plugs and sensors, the upstream sensors that are closest to the engine as they control fuel mixture.