You should use D and Overdrive all the time, except...

You should use D and Overdrive all the time, except:

(1) When you are going down a long deep slope, use a lower gear ("S" or "L" as some individual cars using different terms), to let engine help brake the car.

(2) When you can't pull out from a snow ditch, or a muddy hole, use a lower gear.

(3) When you are towing a trailer and climbing a mountain, you want to increase the towing power consistently and drive slowly, you can use lower gear.

(4) There may be other cases I forget, other people to continue to add here... Regularly, you just do the simple thing - put the gear on D and leave the overdrive on.