I got a K&N CAI and my car tuned, but my car gas mileage according to the fuel screen has decreased. I haven't been driving my car any different then before and have actually been using my brakes less by switching to step tronic and down shifting.

 

Try driving exactly how you did before without the step tronic and downshifting. downshifting uses more gas than regular driving.

 

Try having a true CAI, test pipes, full exhaust, and a tune.
If your car is still going 300 miles on a tank your fine lol

 

My friend's GT-R gets better mileage than my G.
He gets 22.x in his car.

 

^+1 what he said.
all bolt ons and a tune im at 14-15.5 mpg.

 

^ wow, that's pretty low MPG. I have a 6MT and 2WD stock, and I average 21.5mpg in mostly around town driving in summer. Less in winter, like high teens. And I still wish my car got better mileage. On highway, I can get 24.5 or so cruising 70-80 mostly. Car is geared too short to do much better than that. I suppose if I drove 60MPH on hwy, I could eek out 26-27 maybe.

 

I'm around 19 mpg while driving in M mode.
If I put it in drive and behave myself, I get 22 around town and 27 on the highway.

But, I'm always in M mode so I'm always getting 19.
I don't know how to behave.

 

Thats exactly what i have seen... 27 might be on the high side for highway for me.... maybe i need to stop going so fast

 

He fails then, why own a gt-r and drives it like a grandma? Our gt-r is averaging 12 right now.

edit: it my 2007g35x i get 18.5 average daily driving.

 

Downshifting to keep from using brakes is just silly. Unnecessary wear on the engine and it wasted fuel. Just use your brakes, that's what they're for.

Also, a "tune" 99% of the time is to make more power at full throttle, not to get better fuel economy at part throttle.

It is possible that your intake and tune hurt your fuel economy though if the tune isn't right. More likely, it's just that you're downshifting instead of braking.

Side note about the GTR vs. your car. Putting a turbo on a car can make it MORE fuel efficient. But the GTR shouldn't be gettting better fuel economy than yours. The extra weight as well as AWD drivetrain losses and steep gearing kill fuel economy on the GTR.