i have a 2003 trailblazer with 4.2 straight 6 with 73k fuel mileage changes week to week day to day i put a k&n filter kit as i was told it would increase mileage.
no change, sounds better,and i can say it performs somewhat better.its goes from 17-13 around town and
18-21 on road after about a hundred miles at over 2100rpm.tire pressure checked every two weeks im a old fart and dont jack rabbit starts
or jump on it every chance i get.put new plugs and new oem elec. fan recently.everything electronic and
no codes any body got any idea's  

hmm.... that is the same problem the car makers have... trying to improve fuel economy... i have a feeling the problem is the amount of fuel needed to feed the cat to keep it hot and working to put out only clean air...


hows the wheel alignment????   toe in measurment?    can you push it easily on a level surface... brakes dragging a little??? or not????  makes a huge differnce.....

maybe a super magnet in the rear end and thinner oil to lube it...

the super magnet will pull out any metal debris from the oil and hold it...  makes rear ends live a lot longer..    some other companys have used strip magnets... like you find on a fridge door.... you find those  ... nice and clean while you are changing ring and pinion bearings in a 3 years old suv...oops...