Possible mpg gain with header on '06 Altima?
 

My '06 2.5 Altima is 100% stock. I want to find out if I just add a header will it increase mpg's any. The header is only around 75.

A better header can increase efficiency leading to a bit more performance. Depending on how you use that performance could improve your mpg.

On a modern, computer controlled cars, any improvements from a parts upgrade will be fleeting without also retuning the software.

What's"fleeting"? I'm looking at mainly long hwy drives

Replacing the exhaust manifold just gets the gases away smoother. The software controlling what goes into the cylinder remains the same. Okay, that may not get the best out of the system, but it should make a bit of an improvement.

A short time.

The computer has the output parameters of the engine on file. Put a performance part on, and you might see an improvement. Until the software learns the new operating characteristics, and then adjusts engine operation so that it is back in spec.

The ECU software needs to be adjusted, "tuned", to tell the computer that it can take advantage of the better part.

The header is $75, but to benefit from it, you'll need a tune that can cost two to three hundred, or get the software and learn to do it yourself.