Ok, one of the pipes in my header is bent, and there is a small hole in it lettin exhaust gas out. Does does it effect my car if at all?

 

yes...carbon all over the engine bay, hole will get bigger, very hot gasses coming out.
If your hard up for cash you can band-aid it with a fiberglass repair kit...but a hole = thin metal so the entire unit is probably on its way out.

 

would this change my miles per tank?

 

same as the last guy said, you don't want to drive a car with a hole in the header pipe. try to repair it if possible... or just get a new header, they're really cheap now. but it's not too smart to be driving it with a hole.

 

the exhaust thats flowing out there probly means its not going past the 02 sensor. and that makes the engine think its running diffrently then what it really is. so it could affect drivability.

 

^^ thats a good point, makes a lot of sense as to why I im geting bad fuel economy. I am going to try to get new header. Thanks for the help, any other info is still appreciated(sp?)