Let me add a couple of things. Depending where you live plays a big part on what mods you do to your vehicle. I have been in this discussion in other threads.
If you live in a warm climate all the time enjoy your cold air intake. If you live where it is cold and winter almost 9 months out of the year stay away. You need to atomize the air and need it heated for better fuel economy. I will agree that the smoother the air box and associated items better for the air flow.
You can't stop all fuel flow coasting, the engine will stall and you loose power steering, brakes and vacuum. The engine must rotate at least 500 rpm even with the throttle closed. That is what the tps is set to do and the ecm is set to keep spark and fuel going. The way you can stop fuel is if you have a variable system. you can cut cylinders out but you must have at least one cylinder producing power.
The more air you put in the more fuel used. It is a physics thing. The more heat produced the more friction the less mpg. It is also a physics thing. The only way to increase power is increase the piston bore, cc the heads, short stroke the engine, increase the diameter of exhaust, to a certain point. If you decrease back pressure to much you lose HP. There are other things but all these things increase gas mileage.
Heat is an engines worse enemy cause heat causes friction. What does friction cause....decease in mpg.be it rolling friction, wind friction, reciprocating friction. If you look into Newtons three law's of motion. You see how they apply to loss in mpg no matter how you try to over come it.
Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. Or the law of Inertia.
The relationship between an object's mas,, its acceleration*a, and the applied force*F*is*F*= ma. Acceleration and force are vectors in this law the direction of the force vector is the same as the direction of the acceleration vector.
In other words angles play a major roll in decrease in mpg be it change in wind or let's say crank position. Some will be changing, but others constant at certain rpm....
and the one we all know and love
For every action there is an equal and opposite reaction.
In other words just these the forces must be over come to increase your mpg and in reality there are to many constants to over come to make a substantial difference in mpg increases.
I spent the better part of my youth and middle life trying to over come these factors. I know of no one that has made a real perpetual engine, though they have come close to isolated types, til you want it to do more work then it can produce. Like moving a car. Or a engine that will over come it's ability to produce an increase in mpg and have less energy then it needs to produce that increase. Which is actually what a perpetual engine is.
When you talk about removing weight or a roof rack your talking about trying to overcome these principles. Anyone that says they can get 20% to 30% over EPA values need to copyright it asap.
It is possible but you are going to redesign and spend more money then it is worth.
I put in another post in the zj forum about VW tdi engines. One made for the USA one for Europe and South America. Made in the same plant but one gets almost 79mpg. Compared to the USA version that gets 45mpg. All thanks to emission and EPA standards.
So no matter what you try to do to a engine made for North America it isn't going to fly, cause once you go for the inspection your screwed.
I could get into aerodynamics and metallurgy but it's almost 1am and I am missing the end of Sharktopus on the Syfy channel.