The EPA has tested many aftermarket bolt-on parts, NONE has increased the mileage by more than 3%. 3% is their threshold for measurement certainty.

The EPA would nearly mandate a company's aftermarket technology if they could prove that consumers would see a REAL mpg improvement of more than 3% (13 mpg x 3% = 0.4 mpg). I heard this directly from Sid at CARB (those in the industry know exactly who Sid is).

If anybody tells you that their aftermarket part/invention can get more than, in this case, 0.4mpg, then either they will soon be multi-multi-billionaires because the EPA will mandate their invention on all production vehicles or they are merely spreading customers' "rumors" that have zero scientific basis.

If you want improved mileage, the real keys are here: http://www.fueleconomy.gov/feg/drive.shtml

I personally like reducing weight because that improves mpg, acceleration, and cornering.