What can be done to increase the fuel mileage on a 09 Silverado 2500HD 6.0 gas engine?

You have a huge engine, a gas guzzler. You may need to change to a more modern and efficient vehicle - a smaller engine probably. Or a diesel engine???

Lot of thing's Like Tire Pressure, Timing of the Engine, Spark Plug's, Air Filter, Spark Plug Wire's. Gear Rate. Type of Engine Oil it 5w-30 "OR 20w - 50

I have a 2006 Volkswagen Beetle TDI ( Turbo - Diesel - Injected ) has 72,000 Mile's on so-far I'm Averaging 39 M.P.G.'s

they sell a PLUG IN - Performance Gas Saver Tuner Chip Upgrade 2000-2015 (ebay) keeping air filter clean and some fuel additives help. but other than that you would have to change axle gears or you could up the tire size 1 size and lay off pedal will help. try driving and not letting it shift over 2500 rpms. you will see a difference