Better Gas Mileage???

I'm vehicle retarded and just lucked into a 99' C1500 Silverado with the Z71 package. I was wondering if what kind of mileage increases id get with a...

K&N 77 intake kit

aftermarket dual exahust, conservative noise

computer thingy that changes the engine stuff

or a combination of all these modifications

does a bed cover or net tail work?

All of the above modifications should increase your mileage by a little. I wouldn't expect any really huge differences though, maybe 3 mpg or so at most.