Think of D3 as having a 3 speed automatic - 1st, 2nd and 3rd. The car starts off in 1st and shifts normally through 2nd and 3rd. The torque converter may kick in at 3rd - causing RPM's to drop and resulting in some fuel savings.If you shift from D3 to D4 - the car shifts into 4th gear and the RPM's drop further - including when the Torque Converter kicks in.Now, come to a stop and shift into 2 - the car starts off in 2nd gear and stays in 2nd gear until any shift takes place. At stops, the car does not shift into 1st.Shft into 1 - the car starts off in 1st gear and stays in 1st gear. Use 1st only for extreme engine braking or extreme assistance with heavy loads because 1st isn't good past 30 MPH.