Ooops. have you found you're using a lot of petrol? That's because in D you are only using 3 gears, OD is OverDrive, essentially 4th gear. The usual way to drive is to select OD, then drive, the transmission will change up and down thru all the gears as required.
You can manually shift in the lower gears if you wish, though not really necessary. Be carefull when you do though, years ago on my 79 C3 I manualy slected 2nd and in a 'red mist' acceleration thrashing away, forgot to change up, result a massive valve bounce and a very bent valve, leading to a total top end rebuild and regrind of seating and new set of valves!