2002 Trailblazer LT 4x4 vortec straight 6 cylinder. New AC Delco spark plugs, both O2 sensors replaced, Throttle body cleaned, Air filter replaced, drained and replaced all fluids, tranny, rearend, front end, transfer case, engine, all grease fittings greased, tire pressure is correct, ran a can of seafoam through it, all 6 coils replaced, does not burn oil, idles great, runs great, no engine codes, It is completely stock. fuel mileage is a hand calculated 12 miles per gallon in town. It has 162,000 miles on it. Any suggestions? Thank you