[Christ]

You want to use lab conditions right in your driveway? Apply a constant load to the engine at idle and watch for fuel consumption for a couple hours to establish an average, repeat the test with the same outside conditions with the magnets, and then repeat it again without, then again with, etc.

Do this about 16 times for each set of test parameters (with magnets, without) and get the fuel for the tests ahead of time so that you're using the same fuel batch throughout the tests. You will note a loss of fuel economy which can be identified on a curve throughout the test procedure as the fuel ages, but I doubt you'll be recording hourly fuel economy numbers in multi-decimal (4+) formats, so it may not even show up.

Try on a fully warmed up engine using a 1 gallon test tank/cell for 1 hour with no magnets, then again with magnets, then again with no magnets, etc. Keep refilling the tank and noting the fuel measurement required to fill the tank to a predetermined level.

Record data including underhood and outside air temps, specific air density, humidity, etc. They will all be required over the course of testing to discern differences in numbers compared to environmental differences. Too much variation over the test course will invalidate the results.