How much would or does a catback exhaust system effect your gas milage? I am specifially looking at the Greddy EVO 2 exhaust, if that matters?

 

The exhaust by itself really won't make a big difference in your mileage, but your driving style will. Once you have the exhaust you'll probably rev a little higher before shifting and get on boost more often, so you can hear the boxer rumble! The exhaust tone from an aftermarket exhaust on a suby is addicting b/c the sound is so unique. If you get addicted to that sound like I did your mileage will suffer. Now that I have had my exhaust(s) for a while...I'm used to it and my mileage is exactly the same as before.

 

Cool, cause my car is a dialy driver 40 miles back and forth to work everyday, plus heavy traffic in the afternoon, so I am trying to stay atleast somewhat fuel efficient.