Greetings,

I have seen a number of adds for magnets that go on the fuel line and
increase gas mileage by 25%.  Is this pure hogwash?  If so how can they
advertise it and why do reputable stores carry these devices on their
shelves (Harbor Freight sells one)?  If it isn't hogwash why doesn't
every car come preinstalled with one?  (It also claims that it reduces
emissions.)  I understand that many products which claim to benefit
health get by because you can't disprove their claims.  This claim is
testable.  I could measure my gas mileage on a stretch of highway going
65, install the device, and measure my mileage again on the same
stretch going the same speed.  I could idle my car with and without the
device and test emission levels.  

What gives?
William