There is something I don't understand about the relationship fuel trims and mileage.
No matter how lean / richt you set the fuel parameters the oxygen sensors will bring back the engine to run stoichiometric (assuming it's OL). How can you affect mileage with fuel trims?

The only thing I can imagine is that if the fuel trims are positive the engine runs between stoichiometric and slightly lean, and when the fuel trims are negative the engine runs between stoichiometric and slightly rich.
So setting the fual parameters so to have positive fuel trims would save some fuel.

Is there another reason? Can you set a different target for the oxygen sensors? Like 15.5 instead of 14.7?

Stefano