Ok, when I get my V-6 stang next month I am getting upgraded rims/tires for it. Now everyone is saying the offset of the 04 is diff. then the 05 so what are the exact tire sizes i would need to get for the offset, etc.? I plan on getting 18" rims all around but didnt know the size of tires, etc. cause noone wants to order tires/rims that wont be able to be put on there car

 

wheel manufacturers will usually tell you what wheel fits what car.

 

tirerack.com they have wheels for the 05 mustang up to 20" if you please

 

From my calculations if you want to run a wider tire. In 18x9 you will need a +55mm offset. That would make the rim stick out only 3mm(1/16") from stock, keeping the extra inch on the inside of the car. All the 04' rims are 9" wide with +(22-27)mm which will make the wheels stick out close to an extra inch.
But if you want a 8" wide than just stick with the +45mm factory offset and run bigger rims.