No need to mess up your current tank !
I was thinking that the next time that you fill up, you might could re-run a short distance A-B-A-B test, and then just fill up again and start the new tank.
No need to do a long distance test since you have your MPG meter. A test like MetroMPG does with his testing would be cool. ( B.T.W. Does your car have cruise control ? )
I think a lot of non eco-mod people that learn about our methods would attribute it to "driving down a mountain with the engine off ."
A test of aerodynamics done while driving 'normally' might be a good way to convince people.