My friend is looking for a road bike, but can't decide if he wants a double or a triple. He's ridden my bike before, and with it being pretty hilly around here, he needs a good climbing gear. I took him to the steepest hill I know he would climb and had him get into a gear he felt comfortable with. He said it was on the smallest chainring on the front (30 teeth) and the third largest on the rear (21 teeth). So, to decide whether he wants a double or a triple, is there some sort of way to determine what the equivalent gear would be running a 39 tooth chainring in the front?