I regularly got over 20 mpg in my old cherokee with one. of course it was 2wd and standard tranny. when I was 17 I was stupid and into gimmicks and bought one. I passed it on to someone else when I totaled it. In retrospect it was probably a giant waste of money. the only time it would actually affect airflow past the Throttle body is at WOT.
A pretty silly notion alltogether since you buy a K&N to create a faster, smoother airflow
then stick this thing right behind it to generate turbulence.
I have, however toyed with the Idea of scoring my intake ports/combustion chamber to induce a swirl. it seems to me that if you want a good swirling fuel air mix then you should do it as it enters the chamber.
the TB breaks up the swirl, splitting the intake int 6 seperate parts breaks it up, and then the valve breaks it up. Has to be useless, like a magnet on the fuel line.......