Like everyone has said.
Keep it tuned
Check tire pressure. If you are running stock size tires, use the factory recommended tire pressure.
Not the pressure rating on the sidewall of the tire.
Vehicles are designed to get the best mileage with a certain size tire at a certain pressure.
With big tires it gets a little bit more involved.
Eliminate any other cause of rolling resistance.
Brake drag, bone dry wheel bearings, empty rear diff etc..
Make sure the front end is all good and aligned. You can waste a bunch of fuel if the truck is constantly scrubbing the front tires while you steer it down the highway like sailing a ship.
But the biggest impact is your driving habits.
If all is mechanically well.
Drive it like its linked to your bank account.
Which it is.
If you tailgate and stomp on it to pass people, a TBI will firehose fuel down the intake.
But they aint fast, they are loud with a good exhaust tone.
Zen, good music, and relaxation will
get you to the same stoplight that the ******* who was tailgating you gets to.
And you will be right behind him at the same light.
Its a truck, not an ambulance. Drive it like a you would drive a moving truck.
Change your habits and then check the mileage.
Slow and steady will get you to the same place as frantic and freakout.
Just about 1 minute later.