Gas & catback
It's when you add bigger TBs, wild heads, and wild cams... that's when your gas mileage start to suffer. Granted, WOT efficiency is much better than stock(OR else it'd be pointless to do such mods and go race), but part-throttle efficiency suffers because big-power engines are tuned for just that - big power. And to make big power you have to burn a lot of gas.
Good grief! Where'd all that come from???






If you drive the same it will, as they said, stay the same or improve. 