There are a lot of different factors in the answer to your question.
Mileage: In general, advancing the timing will give you the best mileage gains. But there is a cost. Advanced timing will increased cylinder pressure and if you are working the engine hard (towing heavy uphill), significanlty advanced timing can cause too much cylinder pressure and pop a head gasket. In general, the Smarty timing 2 is the do all timing, 3 is for bigger injectors (which functional advance timing - so 3 in less advanced than 2) and the econo - level 4, which is the most advanced timing. I would not tow or run high boost at 4.
Getting the truck up to it's ideal effecient hp as quickly as possilbe will help city mileage. With the Smarty using a different tq managment will get the truck moving quicker with less throttle (not necessarily less fuel) to its most effiecent hp. However, reducing tq man. will increase strain on your drivetran. Another way to do this is us the TNT software. It has more low end fueling and I saw 1-2 mpg improvement with city mileage with the tnt SW.
The pods can be used to help low end fueling as well. Loading a high hp level at a low % will fuel heavier at the low end acheiveing the same effect as above. HOWEVER, I have seen soot in my UOA (see my current thread - soot in oil!) from some of these adjustments, so there's always give and take.
Adding some rail pressure can also increase mileage as it will atomize the fuel better. But it can increase your risk of injector failure.
So ideal mileage can depend on a lot. Not the least of which is your right foot.
Power: Advancing the timing gives a little power but extending the duration of the injection event is what gets the fuel in. Thats what the different Smarty power levels do. More fuel requires more air and you have a good turbo. The maxium hp your truck will support will depend on many things and what it can handle on the dyno may be different than daily driving and definitely different than towing. This is where you have to have gauges to monitor things. Remebering gauges can be off, so error on the side of caution and easy up your hp to something that is safe. Your hp limit now is your transmission. A stock auto is pretty much maxed out at stock power. If it's a manual you might be OK until you get to SW7-9 before needing a clutch.
With the Smarty in hand, you have a wonderful tool but you also have the ability to ruin your engine/trans if you don't know what you're doing. Take is slow and search some post here. There is a ton of info on these forums.
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