To the best of my knowledge improving an air intake on a gas engine only gets you more power at WOT. It does little or nothing to improve mileage (This may not be true for cars with computer controlled EGR however, as there might be room for some improvement in mileage provided the computer is smart enough). On a diesel you get both more power and fuel economy as there is no throttle.
That said, improvements can be made
I found these articles on reducing intake losses. They are aimed at improving performance however.
The first gets better as you go a long. It starts getting good in part 3. Note its for a turbocharged car.
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links to other parts at end of page
This is revisiting it with a non turbo car.
Part1
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Part2
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Part3
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Part4
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Part5
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Done on an economy car
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Picking your intake location
http://autospeed.com/cms/A_1023/arti...popularArticle
a follow up
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