If the company knows what they are doing, the tune will perform as advertized. It's just most tunes for sale are performance oriented. Any economy improvement ones likely aren't worth it factoring in payback. Plus, they might increase emissions.
The cheap chips seen on ebay aren't worth it, and most likely reduce fuel economy. The majority seem to just trick the ECM into staying in open loop. Power can increase, but that's because the car is just dumping more fuel into the cylinder.
Any gains from the K&N and other intake systems is from going from the stock CAI to a WAI. Warm air is less dense and thus will give a virtual engine displacement reduction by letting less oxygen in per revolution in theory. I believe in most cases the gains are from allowing the engine to warm up quick. Though it can have more effect beyond that with some models like the Saturns.
You can just remove the tube going from the stock air box to the fender or grill to get a WAI without buying an aftermarket air intake.
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