Hmm, well, it depends on the situation. From my house to the interstate I leave it in 3rd because it is a residential area and I never go past 50. I usually leave it in 3rd until I'm well on the interstate (pick-up power) and then shift in D. In other situations I usually put it in 3 if I'm at a red light and I know I'm going to be slowing down somewhere ahead. For lower speeds as long as there are no stop lights and no steep hills I just leave it in D anyway.