It seems to work pretty decent. I did a little testing a few weeks ago coming across I-80 in Utah on the salt flats. Picture 40 some miles of strait, flat interstate. 15.2-15.3 seemed to get the best MPG (monitoring with Flashscan and using bidirectional controls) with my heavy truck. Any leaner and it wouldn't make enough power to push the truck with out increasing throttle and mileage would drop. Going from 14.7 to 16-1, you could feel a instant, significant drop in power and almost worse mileage. You might be able to get more out of a car since you have a lot better air shape with less drag, and I have a much smaller motor then ya'll.

 

I've been meaning to play with that... I'm pretty satisfied with 26mpg but more is always better