Howdy,

I do not believe your MPG will suffer in an appreciable manner. In other words, if you're batting 30 MPG, you're not going to realize 25MPG after a few tanks where you compare and contrast your mileage.

If you hit 29MPG, I am thinking that should be the bottom point-- you might not be able to tell any "real" difference.

Hope that did not sound like a bunch of mumbo jumbo ...