all performing 5.3/4l60e
Dec010974 is your 17MPG city or highway? I get around 17 city in my Thunderbird with 3.23 gears and never have a tank that doesn't have at least one full throttle run.
The best way to get economy is make sure all the little mechanical things with the vehicle are up to par like spark plugs, wires, tire pressure, the brakes aren't dragging, all the engine's accessories are working properly, etc. Excessively large wheels, bad alignments, slipping transmissions, steep gears, tall tires and low tire pressure all kill fuel economy as do (some) damaged body panels, poor driving habits, poorly routed exhaust and bad engine combinations. If everything above is in order, the tune would be the last thing I would check and adjust. Adjusting the tune would take a lot of data logging with a wideband O2 sensor and a smart tuner riding with you. A mail-order tune will get you running, but a local tune is the only way to get all the details right.



