I tell people this all the time about tuners and the like but I can't compete against ads that scream otherwise. Detroit truck makers would love to get better mileage for bragging rights and to meet federal standards. If adding K&N filters, intake changes, exhaust changes and tuners could achieve this - without negative results, they would have been all over it years ago. Even when people claim they are getting better mileage, it's hard to believe because there are so many variables in real world driving like air temp, wind speed and direction, tire inflation, etc. Manufacturers on the other hand test under laboratory conditions, so as I stated earlier, if there was any gains to be had, they would have proved them and been all over them.
None of this is to say you can't get other gains changing out parts. A free flowing exhaust can lower exhaust temps and tuners do create more power, but nothing is free. Tuners will void your warranty for a reason. If used sensibly, I doubt they will have a major effect on engine longevity, but sometimes it's hard to drive sensibly when 500+ hp beckons your lead foot. And of course when that happens, mileage suffers.
