How do you increase the gas mileage on a 2004 5.3 liter Chevrolet Silverado I have tried air filter changing the mass air flow sensor and updating the PCM and still get only 16 mpg.?

Well, first off it's a truck so it's not going to get the best mileage in the world. However, here's all the usual things. Does it need a good tune up? self explanatory How fast do you drive it? slowing down saves gas Jack rabbit starts in town? easy on the throttle, it helps How's your tire pressure? low tires use more gas Sixteen mpg's doesn't sound unreasonable to me for a truck but if it's rated a lot higher, say twentyfive mpg's or so you might want to have it looked at. Silverado's have never been considered an economy vehicle. That's probably the best mileage you'll get without seriously changing your driving style.