The usual engine/tune mods
My truck has been an anomaly since I got it.
I used to bang down 17mpg stone stock. 3.73 gears, 18" steel wheels.
It now has factory aluminum 20's on it.
Key things: service the brakes make sure the pads aren't dragging.
Make sure tire pressures are correct, axle levels are correct, parking brake isn't dragging.
Run the highest hp tune you got as well.
And make sure air filter is clean.
And A/C on or off doesn't matter. I turn the air conditioning on about march, and shut it off about October lol.