HOW CAN I GET BETTER GAS MPG OUT OF MY 2003 ENVOY XL

They are not known for fuel mileage, but I would recommend that you dont "jack rabbit" for stops, drive the spped limit, keep it tuned and stay on top of all oil changes, you might want to look at a better exhaust and air intake, this will help make the engine more effient. the 5.3 get 13MPG city and 16MPG on the highway, you should see an average of about 14MPG mixed driving the 4.2 dosent get much better mileage. They are heavy trucks and they were not designed for fuel mileage. here is a link for your fuel rating http://www.fueleconomy.gov/feg/calculatorCompareSideBySide.jsp?column=1&id=19161

I have been reading that going 100% synthetic (oil, trans, and rear,) will make a difference by decreasing friction and it's supposed to last longer. I cannot find any real data on how much more efficiently. So far, I have changed over and am running full synthetic motor oil and have noticed no difference! Also someone on this site recommended Amsoil products. They make all three (motor, transmission, and rear.) With a 6 cylinder, we should be getting 18MPG on the highway. Hope that helps!.