How to know if wheel will fit, Wheel fitment questions Options

[dudeofchaos]

I'm currently sitting on oem gt wheels. But I was planning to get 17x7 this summer. I know they fit because there's a lot of room.

But I will eventually change wheels later in my project and I wanted to know how do you calculate your wheel fitment? Because I will probably make it sit kinda flush.

So the question is: how can I know that I need to buy spacers of ''this'' width to make these wheels/tires flush, how can I know it won't rub, how do I evaluate the tires strechness on wheels, how do I get to know all this?

Thanks.