i'm in the process of replacing my oem wheels for my '08 sedan but i have a few questions about how everything fits together. if i already have 245/45/18 tires how would they fit differently on 18x8 inch wheels as opposed to 18x9.5 inch wheels? would the tires be too small to fit safely on the larger wheel? also, does the offset of a wheel tell you how far the wheel sticks out from the body of the car or how much space is between the inside of the wheel and where its mounted?

one last question: for those who went from stock 17's or 18's to 20's, did you notice a rougher or noisier ride after the switch? i'm considering getting 20's instead of 18's but i lean towards comfort more than performance.