The 10-16 was a guestimate. Since I got 8 mpg overall city/highway I guess that I was doing more city than highway, so I figured that's what killed my overall mileage. I are smart, yet math no my good thing. hehe

 

do both the o2's affect gas mileage or just the upstream one?

 

Just the upstream unit, it helps the PCM control the air/fuel ratio. The downstream unit just tells the PCM if the catalytic converter is behaving or not. If you don't have a CEL with a code for the downstream sensor, leave it be.

When was the last time you did a general tuneup of the vehicle (plugs, cap, rotor, wires)?

Just because the test is good doesn't mean the plenum gasket is good. So many times on this forum have people did the dealership specified test, the test said everything is fine, and then they opened her up and found a slow leak or worse. Sometimes its correct and the gasket hasn't blown yet, but not usually.

 

I haven't done a tune up yet. I got the truck about a month ago and I've been chasing emissions gremlins ever since I got it. Now that emissions testing is behind me I am slowly getting into doing everything else. Going to do that next month along with the plenum gasket/o2 sensors.

 

When I first bought mine about 7 months ago, I was getting about 10mpgs. Then I decided to put the truck in mud up to the headlights. Turns out the previous owner decided not to tighten up the plugs like at all... when I did a tune up I started getting 10-11 again... Now with new O2 sensors, plenum, plugs, wires, cap, and rotor, I'm getting around 12-13...