2003 Mitsubishi Lancer OZ Rally Edition
 

I was woundering if I anyone can help me. I want to put a bigger muffler on my car. will it effect the engine performance or the gas mileage in a negative way at all?

Likely, yes. The exhaust needs to closely match the intake in order to realize a significant benefit. Also, what else have you done to improve performance? If you haven't done anything yet, I would strongly suggest that you get an AEM intake from Road Race Motorsports. This will not only give you a noticeable improvement in power, but will also make your engine sound significantly more bad ass! I did this install in about 40 minutes, with very few tools. An intake is likely the best hp/$ mod you can do.