ummm i dont think auto has "1" gear. because if auto had "1" gear , then it would be equally fast or even faster then a manual car.
therefore , auto has only 4 gears which is "2" , "3" , "4" , "5".
d2 mean it will not shift further then 2 gears (3rd gear)
just drive "d" all the time. its safe for u and for the gears.