Well it depends on where and how you are driving. If you are staying below about 40 or so, D is fine. Also if you are goining up steep hills, or constantly varying speed stay in D. On the freeway, or longer stretches of road over 40 I´d go into OD. The Main reason to keep it in D is to keep it from constantly switching in and out of overdrive and causing undo heat build up in the tranny.