Funny. Why people don't generally offer percentages when talking about FE improvements/decreases is a mystery.
Ah! % increase is more vague than "Up to X MPG increase". It's the "Up to" that sux! "Up to" allows all kinds of magic numbers to be claimed...legally, too!
The average joe sees "Up to" and automatically his "conditioned" (read brainwashed!) mind sez "Oh, I can do that! I'm special!" Right! Joe is a consumer...fair game for every P.T. Barnum trick ("There's a sucker born every minute!") in the book!