Its not a waste of money, cause i dont pay for my fuel :P . I run 91 in my 2006 saab 9-3 and my jeep. I cant see how anyone would be able to prove a difference in mpg using each. There is no way to have the same variables between the two tests. ( wind, temp, terrain etc ) I think it runs better on 91, so thats all that matters. Even if i was paying for it, my 91 is only 5 cents/litre more anyways.