Generally speaking, no.
In fact, those gas saving devices come in two basic categories: completely useless and absolutely illegal.
The useless ones are just what you’d expect: They don’t do much of anything, other than separate you from your money. They won’t hurt your car’s performance, but they won’t help it either.
The illegal ones are a bigger issue. That’s because today’s cars drive along a fine line between fuel economy and emissions. Pretty much any change you make to increase gas mileage will also increase emissions. The EPA calls that “tampering,” and it can come with a hefty fine.
While your local Cottman center will be only to happy to help you keep your car running at peak performance, they won’t get involved with tampering: The risk just isn’t worth anything you could hope to save.