What are 45 or 55 series tires? I've never heard of such a thing lol
And I really couldn't tell you how wide they are, how could I find out? I'm guessing they are on the rim somewhere...

So I can fit wider tires on the 18's right now? So if I have 225's I can put 245's without a problem? Sorry I'm new to this lol

And I'm still clueless about offset :P