So after doing a little reading i think a damaged o2 sensor might be the reason being my horrible mpg's also reading another helpful thread on Zilvia it says there are 2 different kind for the red top, the black top uses the z32 tt. from what i read i need "skinny" version for my red top can someone else confirm before i spend $90 on one.
Here's a link to one i found on ebay.
Genuine Nissan O2 Sensor Skinny SR20DET sr20 Silvia s13 180sx JDM 22690 50F00 | eBay