Originally Posted by bwester04
Use the "D" selection for towing only; for optimal fuel economy and less wear on the tranny using the "OD" selection is highly recommended.
X 1 on this one
only time the truck should be in D is when towing or if you have a very heavy load, for normal driving use OD
I think there has been many many threads on this, you may want to do a search and you might find more info on this as well