What year motor did you swap in? If its a 77-78 motor you most likely will have either a 14 or a 15 tooth sprocket. How many teeth are on the sprocket from the old motor? To figure out your ratio divide the number of rear teeth by the number of front teeth. If your rear sprocket has 48 and the front has 18 you divide 48 by 18 and get 2.67.