how do i get better mpg on my 08 silverado

Get rid of extra weight,keep the engine tuned up, use synthetic oil, keep correct tire pressure, drive very carefully, re-flash the ecm, run a smaller tire/wheel, or change the gearing.

I've found that by running the tire at 45psi, replacing the stock air box with a cold air kit and reprogramming the ecm to tighten up the shift and lowering the shift points I get an average fuel milage of 19-19.5 mpg up from 16.5- 17mpg in my 07 Z71 Silverado.

what are the best shift settings to adjust the ecm to?

K and N air filter, Get rid of the stock muffler, Summit or any after market to make the engine breath easier. spare tire take off thats some pounds eliminated. Use Synthetic and change quality filter and oil at 10 g. Guranteed 2-4 more per gallon in mileage.