hahahaa, i know that feeling.
i've got 16's on my focus and standard size for them is 205/50's but i've twigged that 205/55's are a fair bit cheaper and are gonna be better for daily road use so i'm gonna stick them on a set of alloys for road use and keep my oz wheels for track tyres which will prob be 205/45's.
i will never be happy with one tyre size