Originally Posted by MR.nismo

IMO anything below 20 (in a Z) is abnormal especially since some of you have stated "normal driving". Even with basic bolt-ons and no tune, it should not hinder your MPG's that low. Now if you have installed an aftermarket exhaust and just to hear it roar you get on the gas anytime you get the opportunity, while still "normal" for you, it does not fall into the category of normal "normal" driving habits.
This reminds me when I was younger I had an aftermarket exhaust and a vented blow-off valve installed thus my MPG's fell to the floor. For a while, I was thinking that the "extra power" was making my car more thirsty. The reality is that later on in life, I realized that I was driving like a mad man since I wanted to hear at all times my new exhaust and blow-off valve combo and I wanted the entire town to hear it as well. So it was not the "extra power" what was burning more gas but it was my driving style since I was driving harder than normal just to be loud and obnoxious, but to me that was still considered "normal" driving.
Cheers!!!