2007 5SPD Sport here... I've noticed at highway speeds, particularly 75mph/3500rpm and up, fuel mileage begins to suffer. The engine is 10x louder and working harder at these speeds. I was wondering if a CAI (something like the Injen) would help ease the effort? I do a lot more highway driving than city so I'm not overly concerned with the low end response a CAI would take away from.

 

75 is too fast for best MPG. CAI won’t help with that. Lower your speed.

 

If you're most concerned about freeway mileage, you'd probably get better results with disc style wheel covers. Probably only one or two mpg, but I don't think I've ever heard of an economy gain from a CAI.

https://ecomodder.com/wiki/Smooth_wheel_covers

https://hubcapmike.com/racing-disc-1...ew-on-hubcaps/

 

3500 rpm and ecu has already activated second cam profile and more fuel will definitely use to keep up with the speed,,regular cam profile for economy driving will just be at 3000 rpm.

 

This.
You'll still be at 3500 rpm, the intake isnt going to change the gearing. Drop down to 65mph and enjoy the mpg's.