The Car Talk guys on NPR had an explanation for the success of gas saving gizmos. The buyer wants to believe that his investment will actually improve his gas mileage so he drives with an extremely soft foot on the accelerator and begins to drive very carefully. He may even fudge the data slightly or be selective in his choice of data.
I have noticed what I call the Prius effect. The Prius has a display which shows long term and instaneous MPG. When the little MPG bar spikes, the driver gets a frission of delight at (generally) her part in saving the planet. The conditioned Palovian response is to drive in a manner that maximizes MPG in addition to what you already get by driving a hybrid. (Hybrids are the real deal by the way if you want high mileage. They aren’t really cost effective until gas hits $6.00/Gal.)
In scientifically controlled tests, almost all of these commercially available gizmos actually produce worse MPG, some even have undesirable side effects. You can certainly improve MPG by ripping out pollution control equipment, or making other ill-advised modifications (these are generally not commercially available) that comprise engine life or safety.
I guess if you never try anything you’ll never know...