These have been around for a long time:
http://www.epa.gov/orcdizux/consumer/reports.htmSpecifically, over the years EPA tested the following magnetic devices:
Petro-Mizer, Polarion-X, Super-Mag Fuel Extender, and the Wickliff
Polarizer. None were found to have any measurable effect on fuel
consumption.
The report on the Petro-Mizer quotes Professor John C. Hilliard of the
Automotive Laboratory at the University of Michigan discussing another
magnetic fuel-saving device called the Moleculator. Professor Hilliard
said, "Hydrocarbon fuels such as gasoline have hardly any dipole (separation
of positive and negative charges), and for this reason, the fuel molecules
would not align appreciably in this type of device. If they were aligned,
the fuel dipoles would certainly be randomized subsequently - if not in the
fuel line, then in the process of vaporization prior to actual combustion.
Furthermore, even if such an alignment device did what the Moleculator's
manufacturer claims this one does, there would be absolutely no advantage to
any aspect of mixture preparation or flame propagation relating to
combustion efficiency or vehicle fuel economy."