my 95 850T would get 21-22 in the summer and 18-19 in the winter - which I attributed to winter fuel blends used in New England. Tire brands won't make that much of a difference - assuming you keep them at a correct pressure (ie 36 PSI)

 

Well, downstream O2 is exactly that. When O2 sensors get old, they drift rich. What can the car do except burn more gas?

 

 

 

the upstream o2 sensor controls fuel trim, the downstream is used to compare O2 pre/post cat to measure cat efficiency.

 

That’s exactly what I’m saying. My fuel trim is perfect there’s no sense in changing the O2 sensors I don’t need them for inspection in my state.

 

Often mileage is a perception unless is it checked regularly. If you have the data, graph it and see what it tells you.

Unless the vehicle is running seriously rich, you are not going to see a 20% drop in mileage. The vehicle's computer has enough default memory to not just dump fuel down the tailpipe regardless of the fuel trim. The only way to dump that much fuel is a stuck open injector.

If there are no codes and no irregular data points, then there is no problem.