When people replace the o2 sensor on their car to get better gas milage.... which o2 sensor. Is it the one on the exhaust manafold or the on by the cat? I'm only getting 150-160 per half tank. I dumped a can of gumout into the intake track and tb, it didnt help as much as I hoped but it didnt hurt. on a side question, would installing a turbo increase or decrease milage with the same driving habbits? -sean