Hello,
I was looking into getting the "exact match" for the circumference of our tires in a bigger rim size.
When I went to a Firestone dealer, he said, "those sizes don't exist". So, I'm assuming they only make aftermarket tires in sizes that are the same as factory sizes. And, with that being said, there must not be any cars that come w/ the numbers that you are looking for (245/50-17 and 245/45-18). I don't know about the 18" tire, but I'm sure I was looking for the same size 17". Therefore.....U must change the size a fraction, and I'm sure someone will chime in, and tell you what the affects will be.
Later now,