Nobody ever understands what Octane does.
The higher the octane the higher the ignition temperature. Some vehicles, mainly older ones, require a higher octane due to pre-ignition because of how engine heat is concentrated in areas, or to stop it from igniting in the cylinder (again, because of heat) before the spark sets it off. That's it, that's all. Nothing magic about a higher octane unless you've changed your engine/ignition system in a significant way that makes it require a higher octane.
Proper truck geometry will net the highest gains. When they are tested in the wind tunnel and configured for aerodynamics they are flat, as in no tires and held on a lift/table. When they are sold they have that rake added because of the suspension which is meant to carry heavy loads in the rear. Just installing a level kit added almost 1km/liter to my mileage. I've been tracking it for a long time via fill ups, the trucks trip recordings and calculations.
I went from 12.4L/100km to 11.6L/100km (22-23MPG to 25-26MPG). You can call me out on this if you desire, but I don't care. I'm passing along my experience.
- A tonneau cover will help mileage on the factory suspension because it covers the open bed which destroys any aerodynamics when the trucks ass is in the air. When level it doesn't do a lot.
- Alignment : if they aren't tracking properly this can really affect mileage
- Tires : Not a larger tire but more the tread pattern will affect MPG. I went on my older vehicle (AMC Eagle with full Jeep drivetrain swap) from a street tire to a luggy as hell AT tire and lost about 1/5 of my mileage. But meh.
- Accessories : Ever see those ugly ass fender flares that some places sell that stick out 3-4" to cover the tire? Morons with huge spoilers on their Cavaliers and Sunfires that only work if you're capable of 170 miles an hour? Yeah..may as well add a parachute to the rear.
- Lift kits : they can affect mileage but only because of the larger disproportionate tires needing more horsies to turn and causing a bit of wind resistance. The lift in itself leaves vehicle geometry the same. However if the vehicle was designed to squat at higher speeds a lift kit will ruin it...but that's not what a pickup was designed for.
- Vehicles learn how a person drives and over time will adapt to this for the best MPG. If you swap drivers this can affect it initially but over time it will adapt.
In the end your driving a 5000 pound 6' tall brick, with great capability and usefulness. You'll never achieve amazing mileage