bad gas mileage

[snoopy1]

Recently I decided to fix my check engine light. My O2 sensors were bad, so I changed the pre-cat sensor. Originally I was getting about 250 miles to the tank, after I changed the sensor I got 295 to the tank.

I knew the other sensor was bad, so I changed that a couple of days ago. Now I am getting around 220 miles to the tank.

How does that happen? I'm kinda confused, can anyone help me out?

 

[MAKG]

You put less gasoline in your tank the second time.

That's a VERY inaccurate way to measure fuel mileage. Measure the amount of fuel you actually put in.

Your downstream sensor doesn't affect fuel mileage.