[Piwoslaw]

Here's my proposal of how to test whether magnets help FE: Narrow down the parameters as much as possible.
Does putting magnets on your fuel line effect aero? No.
Does it effect rolling resistance? No.
Does it effect electrical load? No.
Does it effect transmission losses? No.
Then factor these out from any comparison tests!

Magnets are supposed to improve combustion efficiency by "magnetizing" the fuel molecules, so test a bare engine* on a dyno - at all of the rpms and loads seen during driving. Test at different ambient air temps and pressures. Check the fuel used by weighing it. Eliminate as many factors not related to the magnets' properties as possible, and what is left will tell you the truth.

*) To make this a blind test, the engine should not know whether the fuel has a magnet on it or not.