I need some help, please. Does anyone know if a 18x10 with a 45mm offset will fit inside the well of an 05 GT, and what tire size would you recommend in an ET Street?
There is many post's on this and there are many threads in the sticky about this. You should be fine with anything less than a 11 inch wheel, this without making any modification to your mustang. There are many 11 inch wheels that will fit as well, although some require relocating some suspension components. As far as tire size, it would appear that most like 295/45's on a 10 inch wheel.
In the future do yourself a favor and read through the sticky first (The Ultimate Sticky) . This is prolly the 10th "will this tire fit my mustang" thread this week. This way you won't have to wait for an answer as you can find it yourself. Plus, you won't get annoyed responses from regulars.
That is the same size/offset wheel I have on the back of mine with the Nitto 295/45 and it fits fine. I see MT makes the ET Street Radial in a 305/35-18(27" tall) and will fit nicely on that wheel.