I have recently purchased a 2005 Silverado 2500HD 2WD 6.0 Liter with very low miles. I had a bad oxygen sensor and replaced it. I unhooked the battery cable to reset computer but after running several tanks of gasoline through the vehicle, I still only get 9 MPG! The vehicle may have been recked even though carfax doesn't reveal anything.I thinking that something is a little off here!I have a 2002 2500HD 4WD 6.0 liter that gets 14-15 MPG! Please help I would like to know if something needs to be reset and or adjusted.