As to air intake path REMEMBER the engine does not suck in air. The Earth's atmospheric [barometric] pressure forces it into the engine void created by the expansion of the exhaust gases out of the cylinder.
The engine does not create a vacuum just a slightly lower than atmospheric [29.92" HG] pressure.
A highway cruise [where you seem to want improved MPG] the O2 sensor control the mixture and are the limiting factor.
Most of the loss in MPG comes from the fuel not being up to as designed standards! The fuel has less BTU per gallon so more of it has to be burned to create the same speed [RPM].
Oxygenated fuels trick the O2 system into adding extra fuel to maintain the designed AF ratio. If a fuel contains 8% oxygenates, the ecu adds 8% more than designed fuel to try to balance the reading.
Oxygenates are added because they fix emissions on old gross polluting carb engines not because they help modern injected engines.
We are all paying for the old clunkers on the road instead of forcing these vehicles to be retired!
Modern [2002 and later] engines now have expensive WIDE BAND O2 sensors that can sense oxygenates in the fuel and compensate.
Notice that the new 4.5 gets 1-2 mpg better highway mileage......this is because of new O2 ecu design.
If you were to find some magic that allowed more air in you would make matters worse, as the system would have to add more fuel to keep things in balance.
The major fuel hog in modern cars are the tires [which have the greatest friction loss] on the highway. Tires that are good at braking and handling are usually bad as straight highway tires.......good wet weather tires are bad on dry highway.
http://www.energy.ca.gov/repor...2.PDF
If you study the above you will se that oem new tires that came on vehicles are more fuel efficient than even the same NAMED brand you buy as replacements.
Notice the diffence between 30 psi and 40 psi is 1 MPG!
http://autos.msn.com/advice/ar...0utes"They operate on a special laboratory fuel called indolene clear. It's a test fuel that's free of the variability in quality that can be found in gasoline, and it has just eight carbon atoms. As the vehicles run, emissions are collected from the tailpipes. Because the carbon content of the fuel is known, technicians calculate fuel economy by measuring the carbon compounds expelled in the exhaust.
Automakers conduct the tests, using pre-production prototypes of their upcoming new vehicles, the EPA said. You can bet they make sure that anything that would lessen fuel economy—such as underinflated tires, misaligned wheels and poorly tuned engine—are fixed before the test.
The EPA, which announces the top vehicle in fuel economy for each new model year, does its own testing in its own lab—to confirm the numbers from the automakers—on only 10 percent to 15 percent of new models. "