o2 MPG question

[JimRummy]

Ive gone all through the list of replacement basics to return from 15 mpg back to the 20-22 mpg I used to get. Anyhow I already have an o2 sensor I could put in and see if it effects anything, but since I passed smog with flying colors, I have been reluctant. My question is which sensor is the prefered to replace, should I hold off until I absolutely have to, and is it better to do both at the same time?
thanks

 

[adsm08]

What do you mean by "which one is the preferred one to replace"?

 

[JimRummy]

What do you mean by "which one is the preferred one to replace"?

Like before or after the cat.

 

[adsm08]

Upstream will do more for fuel economy. The downstreams are only for monitoring the cat.

 

[JimRummy]

Upstream will do more for fuel economy. The downstreams are only for monitoring the cat.

Ah, good deal. Thank you sir.