Just one more reason I dislike automatic gearboxes, drivers only learn how to put it in Drive and go. I know some of this may sound overly basic, but the decision of when to take it out of D is up to you. D does all the thinking for you while cruising around, but sometimes you just need more control than that over how long the computer holds the gear you're in or when to shift- like when going down the highway in that long, downhill corner...that would not be the time to be in D and finding the computer upshifting. You want to use 1, 2, and 3 for those times you need to control engine or vehicle speed more closely. In 1, 2 or 3, you use the engine braking more effectively. You may be coming up over a rock (or curb) and want to slow going over the top of the high point then go again, maybe 1 gives you the control to hold the transmission in gear so you aren't coming in and out of gear right when you need to be in the power again without rolling backwards. Going up isn't as demanding on gear selection as going down, the auto will upshift and downshift as necessary to the highest gear selected automatically whether 1st, 2nd, 3rd or D (up to 4th/overdrive). Going downhill it is the reverse, staying in D lets the car "freewheel" down hills where 1, 2 and 3 limit the upshift to the gear chosen and will hold the gear more before letting the torque converter freewheel. Putting the transfer case in 4Lo engages a different set of shift parameters and the transmission will hold gears longer, upshift later and have a firmer, more aggressive shift feel at the same time. I guess after saying all that, 1,2, and 3 in an automatic are the closest you will get to full control over your vehicle like you would with a manual transmission. Use the lower gears to control engine and vehicle speeds, and shift up/down as needed for the conditions.
