I got the best mileage out of my 03 GMC 1500 today. 21.5 on the way to Belleville and 18.4 average round trip, had a bed full of superduty seats on the way home so I added a couple hundred pounds and a full tank of gas. About 65 MPH roundtrip.
I put Lucas Fuel treatment in it a month or so ago and this is the first I have driven it other than short commutes. It was getting 16-17 empty and 12ish towing a car on the trailer.
It also free'd up the check ball in the fuel pump, used to have to cycle the key to get pressure for it to start right away.
I bought a gallon of it and have been putting it in everything.
I really like the GMC again.
I am going to try and find a factory electric fan to swap in place of the clutch fan setup it came with.