A big misconception is that running very lean will return high mileage. That is false. The best mileage will be returned on the AF ratio that provides the most power output with the least fuel injected. Running lean lowers power and causes you to have to use more throttle to maintain speed. As a result of the extra throttle, More air is used along with more fuel.
Some tuners lower the Closed loop RPM or throttle positons to levels that are way to low. As a result, You guys drive around in open loop and that returns poor mileage. The correct settings should yield 23-25mg or so driving the car around 3500 rpm and staying out of boost. More on a highway trip using the cruise.