How much do aftermarkets affect gas mileage?

I have a 03 Z71 with a 5.3L engine and am thinking hmm. Gas mileage is not so great. How much would a cold air intake, exhaust and a programmer improve my mileage? Would that money spent be worth it?|||probably 5%. You can reduce pumping efficiencies and make more power. As long as you drive normally instead of faster with aftermarket parts on, gas mileage will increase a little bit especially if you tune the ecu to run a little leaner, but be careful to not make the engine knock when you lean it out if you do.|||cold air and exhaust stuff usually just gets more power out of the gas you use, though it will improve it a little. a program chip is better but more expensive.