Rich,
Honestly, I would account the improvement moreso that you drive SLIGHTLY different then the device actually working. Extensive tests from different magazines have been done. They all came to the same conclusion. it doesn't work and some vehicles actually had a loss of power with it installed.
They try to convince you that the air will swirl while entering the combustion chamber. The problem is that vortex generator is before the throttle plate. Any vortex created by the generator will be slowed down by the plate. Then take into consideration that it goes into the intake manifold wwhere it is split up between 6 tracks for the air to travel to each cylinder. Also air, like water, takes the path of least resistance. A swirl is more resistance then a straight smooth line. To get more power out of the same displacement without forced induction is smoothing the intake. They do that by porting the manifold (the make them as close as possible the same size) and then polishing it. That will allow the air to flow as freely as possible to increase power by making the engine more efficient. The more efficient the engine is, the more power it will produce. The more power it produces with the same amount of fuel will allow you to use less fuel to produce the power needed to fight wind resistance. Another thing they will do is a multi angle valve job. 5 angles will allow the air to enter the combustion chamber even easier. Also, going a little larger on the intake will allow the air to enter more freely. The Vortex generator also blocks your intake a little.
There are too many things that disagree with the theory that the vortex generators don't work then those that do.
I have been driving my Trac as sane as I possibly can. I use my ScanGuage to average my MPG's as I drive. Going to work, I can average 21 MPG. That is in traffic at 3 in the afternoon. At 11:30 when I am driving home (I work 3:30 PM to 11:30 PM) I can average 23 MPG. Driving in the city with traffic lights and stop signs, I average 15 MPG. If I make it a habit to accellerate downhill then as I am going up the next hill I start to slowly let off the accellerater as I go up the hill, I can maintain higher MPG's then if I coast down the hill then accelerate up the hill. Been trying it out for two weeks now. The first tank I got 17.9 MPG, the second tank I got just under 17 MPG. I had to drive a little more agressive on the last time because somehow I was running late for work a couple days last week.
Tom