I was looking at order forms for chevy th400 trannys because i'm trying to learn all about transmissions. On the form it asks if you want a 4" Large tail shaft or a 9" small tail shaft. It also asks if you want speedometer gears, yes or no. So I'm asking, what is the difference between a 4" large and 9" small tail shafts and what are their applications, and what is a speedometer gear?