As an owner of a 2007 Silverado Classic, I feel your pain. Here are some things that have made a difference for me:
1. Monitor real-time fuel economy as you drive. By doing this, you can learn how your
truck responds to different driving conditions and driving techniques. As an example,
your truck will coast a really long way, so let off of the gas earlier when approaching
stops. You can improve your mileage as much as 25% by altering your driving habits.
2. Air up your tires 10% over the door sticker to improve rolling resistance.
3. Use synthetic oil in your crank case - makes around .5 mpg difference.
4. Adding a tonneau cover will improve highway mileage 1-2 mpg
5. Remove your DRL bulbs to reduce electrical load.
6. Things you can alter with a PCM/ECU programmer:
lower idle speed by 50 RM - lowers fuel consumption by .1-.2 gallons per hour at idle
reduce shift delay just a tad - no difference in mileage but it makes mine shift better
lower shift points by a few mph to shift sooner (normal mode only, not in tow mode)
7. In cooler weather, a grill block allows the truck to warm up faster AND improves
aerodynamics at highway speeds. (adds a couple of MPG at highway speed!). Warming
up your truck faster makes a difference in mileage because your truck's air fuel ratio
is around 12.5 to 1 until the oxygen sensors warm up, and the AFR then changes to
around 14.6 to 1 for closed loop operation.
http://www.truktoys.com/modelView.aspx?id=45
8. Don't let your truck idle for more than 20-30 seconds, including your first start in
the morning.
Good luck improving your mileage.
Wayne