i am planning on converting my Raleigh Marathon to a single speed and have been reading up on gear ratio's. anyway i want to be cheap about it and use one of my existing chain rings and just take the other one off. i have a 52 tooth and 40 tooth on right now. i know i want about a 77ish GI which i can get with either 40/14 or 52/18. is their a difference and if so what is it and which should i go with?