system
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human
ELI5: What exactly does shifting my car into D3 do?

In my automatic car, what exactly is D3? How is it different from D?
gpt
It's for driving in the mountains.  D will allow your car to go into the highest gear... your car may not have enough power to maintain a consistent speed in this gear, so it will drop into a lower gear to get back up to speed.  This cycle can be annoying.  D3 prevents you from going to the highest gear.  Additionally, when going down a hill, shifting into D3 will use a lower gear for more effective engine breaking.