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I bought one and didn't notice anything special but I let a buddy put it in his truck to try it and I noticed that my mileage went down when I didn't run it. It helps but not enough to justify buying one.
You can't swirl air thru a mostly closed throttlebody, so how can mileage improve?? There is no fuel in the intake to atomize unlike a carb or TBI has.
Only if the flowpath is maxed out already. If your engine needs 450 cfm to make peak power and the intake duct can flow 800 cfm it won't hurt it to be restricted a little bit. That's not to say an air swirly thingy is a good idea, just that they aren't that big of a restriction. I doubt you could find any power difference in most vehicles on a dyno.
They aren't on race vehicles, they aren't on OEM vehicles whose makers spend millions to meet CAFE standards, and they aren't PROVEN by a repeatable scientific test to work.
I was working on my 96 FSB MAF and I was looking in to the air tube. I saw something strange. The PO of my FSB put one of those in the air tube. I took me awhile to get it out. I still have it. So, do you want it?
I feel those things are worthless, and I would not give it away if I could.
Just keep your truck tuned up and you will be better off.