I have a GSR swap in my EK hatch and initially, i was running a test pipe. After a couple of months, i got my hands on a JDM ITR cat which bolted up perfectly to my JDM 4-1 and Mugen twin loop. Anyhow, i believe (still on my first tank of gas after the cat and the first half of this tank is when i was still running a test pipe) that my gas mileage has gotten better. Has anyone noticed a difference between in fuel consumption after installing a test pipe?

 

hrmm....i've had my test pipe on for the past 2 years.....only throw on my cat the day i have to smog.......so i really havent gotten a chance to see the difference in fuel consumption

 

it doesnt make sense at all but I have heard several people agree with you...seems like to me it would get worse but what do i know