I'd say 5-6mpg. What gives? I thought this was supposed to better your fuel economy!

 

ECU doesn't like lowered back pressure. Replace the Cat and see if mileage is restored.

 

I agree with the ECU reasoning. You most likely have to tell the car that you removed the CAT, then see what happens. Do you have any O2 sensors down stream of the CAT...if not than just like said, your car doesn't know the CAT is gone and has not compensated for the loss in back pressure....you have a 1.8t? keep in mind the turbo uses that back pressure too. Better get a computer in the car and make some adjustments if you want to keep it CAT free.

 

Yeah, I have downstream O2 sensors. I am shopping for either HF-cats or something else.

It's a 2.8