I installed the svcm 1000 miles after passing smog, I don't think using premium or a defouler would give you better gas mileage. The svcm has taken 2mpg from me but hopefully that lost milage will even out in future repairs it may or may not help. I'll keep you posted. And I still have a extra new bank 1 cat when ever it fails. (Was only 120 compared to a cali carb version for 1200)A long overdue thanks for that reply and references. Did you notice any performance or fuel mileage improvement after installing the defouler + 4 tanks of premium?