Answer:
actually you should have an automatic in highest gear on column as possible that's what the computer is for it determines what gear you should be in if you have a manual transmission then read owners manual it will tell you when the correct times to shift from one gear to the next for fuel economy and engine life
first, not all cars have computers
,second if it is an automatic, it does not matter if car is in overdrive.the transmission will choose which gear is needed for the speed and load
if a manuel transmission, the driver can hear and feel whether the car is in the right gear for the driving conditions