From driving 50/50 highway and city, I have had no noticible change in MPG. I dont realy see why it would drop your MPG. That being said, I did have a OBX header before and after the install, so maybe there is a difference when changing header. I get an average about 28-31 MPG. The only thing I see being different is the 2nd O2 sensor getting inaccurate readings. But that is for the first cat anyway, since A/F coming out of the engine is read by the first O2 sensor.
