my catalytic converter was stolen. Because of this, my car now smells like gas and gets worse MPGs. I do city driving and now after a mechanic straight piped the car, I get about 23 MPGs as opposed to about 26 MPGs. I used to always fill up my tank and reset the trip meter to calculate MPGs the old fashion way. Anyway, my question is: if getting a straight pipe is supposed to improve MPGs, then why have mine decreased? Did the unprofessional thieves do something differently compared to professional straight pipers who would normally yield us those benefits?
Thank you for any help!