I used to get 15.2-15.5 mpg with Injen CAI and Borla TD. After I installed cat (about 1 week) my mpg is 17.5-17.7.

I do have sond that similar to sheet metal virabrating as RPM drop from 3k to 2K. But form 2k up to 3k, I do not have this sound.

Also the sound was there since Borla installed, with the cat, the sound is still there.

 

Dang 15mpg!!! How are you driving that sucker, I'm avg meter reads 22.2 mpg right now, here in cali we dont do much redlining because of the gas prices

 

I do not drive it hard. I can only go about avg. of 25 mph to and from work. I guess at that speed the mpg is not good.

I think other people on this forum also reporting about 15 mpg also.

 

Those are both horrible mpg, geez! I'm getting about 19 road and 22 hwy.

 

If I reset the computer and drive on highway (>65 mph), I got good (>25) mpg.

 

damn for my mainly city driving i get about 19/30 with my injen intake /nismo exhaust.

 

Yeah, I get 21.5 with mixed driving.
How many miles are on your Z?
When mine was new, it only got 17 mpg mixed, but after about 10k, the mileage got better.

 

3950 miles

I also found out that I have a small leak on Borla H-pipe. This is where that rattle noise come from.

I tighted the clam as much as I can, the small gap is still there.

I will take it to muffler shop and have it welded.