For a long time now, I have not been able to figure out this issue. I used to get pretty decent mileage, averaging about 16MGP. Now I get 10-11, and something is clearly wrong, but no real codes. I have a bad cat, constant P0420, which is probably due to a bad tune on that bank, related possibly to this issue. The RPMs under constant steady load say on freeway fluctuates 100-150 RPM, and the instant mileage readout bounces all around. I heard of similar issues with the torque converter lock up going bad, but that is much more pronounced.
Any guidance appreciated..