Jeff, The XIED creates a new reference point. Rather than changing from rich to lean voltage at 14.7:1 you can select other ratios. Once you do the BMSK uses what it learns during closed loop (cruising) operation to adjust Open Loop fueling. It turns out that a slightly richer cruise creates a better roll-on throttle response as well. As far as mileage goes from the richer cruise, about half the added fuel is used to produce power and half goes out the tailpipe. So if you add 6% fuel mileage at high cruise >65 MPH goes down about 3% but for speeds below that you will often run a higher gear and get better mileage.